When BLK is used and a BLOCK CONTAINS clause is
specified, the compiler allows blocking for DYNAMIC access files and SEQUENTIAL
access files with a START statement. Blocking is not allowed for RELATIVE
files opened for output operations. The BLOCK CONTAINS clause controls the
number of records to be blocked.
When BLK is used and no BLOCK CONTAINS
clause is specified, the compiler allows blocking only of SEQUENTIAL access
files with no START statement. The operating system determines the number
of records to be blocked.