There are a number of Format 3 modules delivered with CA Datacom 15.x - primarily those that are a part of the SQL component. The binder for format 3 requires PDSE formatted datasets. Using a regular PDS for these modules (now called Program Objects) will cause the binder to fail.
However, there is no requirement at this time to create Format 3 custom modules, so the CUSLIB can still use a standard PDS. We would recommend that all loadlibs be formatted as PDSE usage for consistency, and to allow for the possibility of a future solution that might require a Format 3 module.