The class condition tests whether the contents of an operand are
  numeric or alphabetic.  It also determines if an alphabetic operand
  contains only uppercase or only lowercase characters or if the
  operand is a class-name.

  Format -

                           { NUMERIC          }
   identifier-1 IS [ NOT ] { ALPHABETIC       }
                           { ALPHABETIC-LOWER }
                           { ALPHABETIC-UPPER }
                           { class-name       }