It replaces or adds to the standard roster with legendary "classic" squads from different eras (e.g., 90s icons, early 2000s giants).
Interestingly, the term "PES 2015 Patch" has recently been used for mods that downgrade modern games . For instance, there is a PES 2015 Season Patch for PES 2021 Pes 2015 Classic Patch
You might ask: Why mod PES 2015 instead of the newer PES 2021 or eFootball? It replaces or adds to the standard roster