A factor is the smallest part of a program that has a value. Most factors in XPL0 are either constants or variables. A constant is a value that remains unchanged throughout the execution of a program, while a variable is a value that can be changed. Factors are classified as integer or real. An integer is a 16-bit value that represents a whole number. A real number is a floating-point value that is not limited to a whole number and can cover a very large range of values. Thus there are basically four kinds of factors: integer constants, real constants, integer variables, and real variables.
Here are detailed descriptions of the various factor types: