An identifier is a combination of letters, digits, dollar  signs
  ($),   and   underscores  (_)  that  conform  to  the  following
  restrictions:

   o  An identifier cannot start with a digit.

   o  An identifier cannot contain any space or special symbols.

   o  The first 31 characters must denote a unique name within the
      block  in  which  the identifier is declared.  An identifier
      longer than 31 characters generates a warning  message;  the
      compiler   ignores   characters   beyond   the  thirty-first
      character.  An  identifier  cannot  start  or  end  with  an
      underscore, nor can two adjacent

1  –  Predeclared Identifiers

  Predeclared identifiers name data types, symbolic constants  and
  file  variables,  procedures, and functions.  You can redefine a
  predeclared identifier, but, if you do, the original declaration
  becomes unavailable within the block in which you redeclared the
  word.  Examples of predeclared identifiers include ADDRESS, COS,
  INTEGER, SQR and TRUE.

2  –  User Defined Identifiers

  User identifiers denote the names of programs, modules, symbolic
  constants,  variables,  procedures, functions, program sections,
  and  user-defined  types.   They  represent   significant   data
  structures,  or  values  and actions that are not represented by
  reserved words, predeclared identifiers, or special symbols.