Enterprise COBOL for z/OS, Version 4.2, Programming Guide

In an RRDS that has fixed-length records, each record occupies one slot.
You store and retrieve records according to the relative record number of the slot.
A variable-length RRDS does not have slots; instead, the free space that you define allows for more efficient record insertions.

When you load an RRDS that has fixed-length records, you have the option of skipping over slots and leaving them empty.
When you load an RRDS that has variable-length records, you can skip over relative record numbers.