I would guess they'll be explaining whatever this is for before the start of the playtest, thus before the 24th of the month, in a direct.
And i think it may be about something about cloud gaming as it's the only thing i can think of that would require such an open playtest as opposed to internal testings.
If it's something like using a Switch to play Switch 2 games on the cloud and they want to test out the infrastructure, and the Switch 2 presentation that would include that info ends up being before the Unity 6 release (october 17th) then it would make sense for such a presentation to be on next tuesday, on the last day of the playtest registration period, with the direct announcement tomorrow or on monday (i doubt they would just throw an announcement at a sudden, after all).
It would be strange if it would something solely Switch related when it's almost at the end of its lifecycle, after all.
Regardless, let's wait and see.