A constant is any word that is neither a variable nor an operator. There are four categories of constants in KIF - object constants, function constants, relation constants, and logical constants. Object constants are used to denote individual objects. Function constants denote functions on those objects. Relation constants denote relations.
Logical constants express conditions about the world and are either true or false.
All other constants are non-basic in that the language user gets to choose the type and the meaning. The user selects the category of every non-basic constant for himself. The user need not declare that choice explicitly. However, the category of a constant determines how it can be used in forming expressions, and its category can be determined from this use. Consequently, once a constant is used in a particular way, its category becomes fixed.