Business vocabularies include terms used by business people for such activities as stating business policy.  Many terms represent categories of things or roles that things play.  This article discusses categories and roles with regard to definition, meaning, and practical use.  The distinction between categories and roles is important when defining terms in a business vocabulary or when using them to write business rules.

A term in a business vocabulary represents a concept.  A term’s definition typically refers to a more general kind of thing and then states necessary and sufficient characteristics that distinguish things denoted by the term from other things of that more general kind.  The set of characteristics represented by a term is called the intension of the term.  Rarely, a definition lists all of the things denoted by a term -- the term’s extension.  However, such a definition is inappropriate for roles and is impractical for most categories.

Category and role are two kinds of concept.  A category is a specialization (or subtype) of a more general concept.  Its intension is stricter than that of the general concept -- that is, the category’s intension includes at least one distinguishing characteristic beyond the intension of the general concept such that the extension of the category is a subset of the extension of the general concept.  A role can be thought of as a special type of category whose distinguishing characteristic is a part played in relation to something else.

Here is a definition of a term representing a category:

physician

person licensed to practice medicine

The preceding definition of 'physician' starts with a general concept (person) and then states the necessary and sufficient characteristics that would qualify a person as a physician ('licensed to practice medicine').

Now consider the following two definitions of 'author':

author1		writer of a book, article, or other text
author2		person who practices writing as a profession

The second definition resembles the definition of 'physician.'  It defines a category of persons based on a profession.  The first definition differs from the second in that it defines a role in relation to a book, article, or other text.  A statement that someone is an author₂ tells the person’s profession.  But a statement that someone is an author₁ indicates that the person has written something -- a fact about anyone who has learned to write.  So 'author₁' is not useful for classifying people.  But 'author₁' is very useful when talking about who wrote a book, article, or other text.  Using 'author₂' for authors of books would work only for books written by professional writers.

To avoid ambiguity, a business glossary that must represent both senses of 'author' might define different words or phrases for the different concepts.

author		{role} writer of a book, article, or other text
professional author		person who practices writing as a profession

Note that the first definition is preceded by '{role}.'  A helpful practice when defining a special type of concept, such as a role, is to mention the type of concept in italics within curly braces at the front of the definition.  Other special types of concept can be shown in the same way.

A role has a sense of relationship often carried by a verb.  For example, consider the following two business rule statements:

1. Each bank policy statement must be written by a manager.
2. The author of a bank policy statement must be a manager.

The two statements express the same business rule.  The first uses the verb 'written' while the second uses the term 'author' to convey the sense of 'written.'  A term representing a role is often used with very generic verbs (such as 'has') because the role conveys the meaning of a more specific verb without the verb being present.  Roles also work with simple prepositions, such as the use of 'of' in the second example above.  More elaborate phrasing is required when not using roles.  Consider the following two phrases that convey the same meaning:

1. person who wrote an article
2. author of an article

Roles are very useful for relationships involving common things such as numbers and monetary amounts.  Some terms that represent roles of a number are 'absolute value,' 'square root,' and 'sum.'  Terms for roles of a monetary amount include 'account balance,' 'transfer amount,' 'deposit amount,' 'debit,' and 'credit.'  These all convey a sense of relationship, either between numbers, or between a monetary amount and an account or a financial transaction.

The key to distinguishing a role from an ordinary category is whether its meaning conveys a relationship to something else.  An ordinary category is often used to simply classify things.  For example, it is meaningful to say that a person is a physician.  It is less meaningful to say that a monetary amount is an account balance.  Any monetary amount might be a balance of some account somewhere, so 'account balance' is not useful for classification.  Its real meaning is in relation to an account.

The following example demonstrates an important difference between roles and ordinary categories.  Consider the following two definitions and the subsequent authorization statement:

patient		{role} person who receives medical attention, care, or treatment
physician		person licensed to practice medicine
A medical receptionist is authorized to determine which physician sees which patient.

Based on the definitions given, the authorization permits a medical receptionist to assign any person as a patient of a physician even if the person was not previously known to be a patient.  But the authorization does not permit a medical receptionist to assign a patient to an arbitrary person as a physician.  Being a physician is not simply a matter of seeing a patient.  But a person treated by a physician is implicitly a patient of the physician.  The same authorization can be stated differently by replacing the use of the role patient with a verb phrase that conveys the meaning of the role:

A medical receptionist is authorized to determine which physician gives medical attention, care, or treatment to which person.

The restatement conveys the same meaning as the first authorization statement because of the way patient is defined as a role.  The substitution cannot be made if patient is defined as a category.  For example, consider the same rule statement but using a different definition of 'patient':

patient2		person admitted to receive medical care and identified by a patient number
A medical receptionist is authorized to determine which physician sees which patient2.

In this case, the statement does not authorize a medical receptionist to assign a person to a physician unless the person is a patient, that is, someone who is admitted and who has a patient number.

The preceding examples all define roles based on more general concepts that are categories.  What happens in the less usual case where a concept (whether a role or a category) is defined to be based on a more general concept that is a role?  Here are some valid examples in the context of a credit union:

account balance		{role} monetary amount that is the sum of credits and debits of an account
savings balance		{role} account balance of the savings account of a credit union member
negative balance		account balance that is less than zero

The term 'savings balance' represents a role that is based on another role.  The definitions make sense because the relationship indicated by 'savings balance' incorporates the relationship indicated by 'account balance.'  'Savings balance' represents a monetary amount in relation to a credit union member and the savings account of that member.  'Account balance' represents a monetary amount in relation to an account.  Defining a role in terms of another role in this case poses no problem because each savings balance is an account balance of an account.

The definition of 'negative balance' picks up the idea of a role from 'account balance,' but its distinguishing characteristic is not about a relationship but about classification of a monetary amount as being less than zero.  'Negative balance' can be used in statements in the same way 'account balance' is used to indicate a monetary amount in relation to an account, but 'negative balance' conveys the additional idea that the monetary amount is less than zero.  The following two statements express the same business rule -- the second one simply replaces 'negative balance' with its definition:

1. A savings account must not have a negative balance.
2. A savings account must not have an account balance that is less than zero.

One final topic that occasionally arises regarding categories and roles concerns cases in which no single concept starts a definition, but rather, a disjunction of two or more general concepts is the starting point.  Here are two more definitions of 'patient' for illustration:

patient3		{role} person, animal, or plant that receives medical attention, care, or treatment
patient4		person, animal, or plant registered to receive medical care and identified by a patient number

As shown above, this sort of definition can occur for both roles and categories.  Patient₃ and patient₄ are based on a general concept whose extension necessarily includes all persons, animals, and plants.  If concepts are defined that are supertypes of all three, then the most specific of these can be considered the general concept that is the basis of the definitions.  For example, if each one of the three terms -- 'person,' 'animal,' and 'plant' -- is defined to be a 'living thing' (directly or indirectly) then the previous definitions are equivalent to these:

patient3		{role} living thing that is a person, animal, or plant and that receives medical attention, care, or treatment
patient4		living thing that is a person, animal, or plant registered to receive medical care and that is identified by a patient number

In summary, categories serve well as classifications of things, while roles represent things in relationships to other things.  Words in natural language are often used in both ways.  Formal business vocabularies should clearly define the sense of each term and distinguish roles from ordinary categories.  Terms representing roles often facilitate natural and concise business communication.

References

[1]  ISO 1087-1 Terminology work -- Vocabulary -- Part 1:  Theory and application, first edition.  International Organization for Standardization, 2000.

[2]  Example definitions are based on The American Heritage® Dictionary of the English Language, fourth edition.  Houghton Mifflin Company, 2000.

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