A field within a structure can itself be a structured item composed
  of other fields, other structures, or both.  You can declare a
  substructure in two ways:

   o  By nesting structure declarations within other structure or
      union declarations (with the limitation that you cannot refer
      to a structure inside itself at any level of nesting).

      One or more field names must be defined in the STRUCTURE
      statement for the substructure because all fields in a
      structure must be named.  In this case, the substructure is
      being used as a field within a structure or union.

      Field names within the same declaration nesting level must be
      unique, but an inner structure declaration can include field
      names used in an outer structure declaration without conflict.

      %FILL can be specified in place of a field name to leave space
      in a record for purposes such as alignment.

   o  By using a RECORD statement that specifies another previously
      defined record structure, thereby including it in the structure
      being declared.