The record type must be present if sequence checking is specified.
O
The record type is optional (that is, it may or may not be present)
if sequence checking is specified.
This entry must be blank if positions 17 and 18 contain an alphabetic entry.
Sequence checking of record types has no meaning when all record types
within a file are specified as optional (alphabetic entry in positions 17
and 18 or O entry in position 20).