is the Alternate Record Key for the file. It is the data-name of a
  data item in a record description entry for the file. It can be
  qualified, but it cannot be a group item that contains a
  variable-occurrence data item.  The data item must be described as:
  (1) alphanumeric or alphabetic category, (2) a group item,
  (3) unsigned numeric display, (4) a COMP-3 integer, or (5) a COMP
  integer.