Is the PlayFab Online Subsystem required for PlayFab + UE4 party ?

Hello

We are trying to use UE4 + PlayFab and port our game to PlayFab.

Currently, we have integrated the GSDK and can run the game on PlayFab multiplayer servers (using a manual "open hostname:port" command.

Now, we'd like to work on the pre-match lobby and want to integrate PlayFab Party for this.

I am a bit puzzled by the exact difference by using the standard C++ Party SDK that is available to all and the additional features supplied by the PlayFab Online Subsystem.

Can I write chat + network functionality with the stock SDK, just using the standard "null" online subsystem?

Also, there is a rather old PlayFab Online Subsystem plugin written by an individual developer (on https://gitlab.com/mtuska/OnlineSubsystemPlayFab/tree/master) and another version that requires registering as an XBox developer. Are these the same ? Did I get this right?

If someone who has to make that decision can comment, we would be very grateful!

TIA