THE ALTERNATE RECORD KEY clause specifies an alternate access path to
  indexed file records.

  Format -

   ALTERNATE RECORD KEY IS { alt-key | seg-key = {seg}...}
                                 [WITH DUPLICATES] [ASCENDING ]
                                                   [DESCENDING]

1  –  alt-key

  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.

2  –  seg-key

  is the Alternate Record Key for the file. It is a segmented-key name
  that represents the concatenation of one or more (up to eight)
  occurrences of seg.

3  –  seg

  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 or (3) an unsigned numeric display item.