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PDS and PDSE/Library are different in that a PDS can contain a limited amount of members dependent on the directory block size. This means that PDS's require compression every so often to free up unused members and the directory blocks they are taking up.

A PDSE/library Does not have this limitation or problem and does not require periodic compression.

For this reason I only define Libraries/PDSE's. I also take the opportunity when available to redefine PDS's as Libraries/PDSE's.