Insertions into a Variable-Length Relative-Record Data Set

A variable-length RRDS is processed in the same way as a fixed-length RRDS, with the following exceptions:

You must specify the record length in the RECLEN field of the RPL macro.
Insertions into a variable-length RRDS use the free space provided during the definition of the data set or the free space that develops because of control interval and control area splits.

As for a fixed-length RRDS, you can insert records into a variable-length RRDS either sequentially or directly.

Sequential Insertion. When a record is inserted sequentially into a variable-length RRDS, it is assigned the next available relative record number in sequence. The assigned number is returned in the argument field of the RPL. Use mass sequential insertion with a variable-length RRDS.

Direct Insertion. Direct or skip-sequential insertion of a record into a variable-length RRDS places the record as specified by the relative record number in the argument field of the RPL. If you specify a duplicate relative record number, VSAM sets an error return code in the RPL and rejects the request.