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       }

1 - identifier-1

  Identifier-1 must reference a data item whose usage is explicitly
  or implicitly DISPLAY. If identifier-1 is a function-identifier,
  it must reference an alphanumeric function.

2 - class-name

  is the user-defined word for a class. It always possesses the
  global attribute