is the 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.