Identifiers

 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

Additional information available:

Predeclared_Identifiers         User_Defined_Identifiers