Internal names are a sequence of letters and digits that represent some value or item. There are three sets of internal names: file names; procedure names; and field, variable, or parameter names. All of the internal names must conform to a common set of rules:
The first character must be a letter (a-z, A-Z)
After the first character, there may be any number of the following characters:
a letter ( a-z, A-Z )
a digit ( 0-9 )
a pound sign ( # )
an underscore ( _ )
Only the first 15 characters of any internal name are used. Any characters beyond that limit are ignored.
The name must not be a reserved word (see the section on reserved words at the end of this chapter).
In addition, some restrictions apply to each set of internal names. For file and procedure names, each name must be unique within the set. For field, variable, and parameter names, duplicates are permitted only among field names, and even here the names must be unique within one file (one file cannot have two fields with the same name).