Cross gen bundle?

Could someone please explain what it ACTUALLY is? I've had a lot of "I guess it' s...... ", or the like, on other platforms but no one has hard facts as to what it actually does.

Zero mentions from the YouTuber's (apparently that's a MUST SEE™ detail non of them have mentioned, or indeed willing to comment on) 

There is no clear description that I, and apparently everyone else ive spoken to, actually know of.

Reason for asking, I'm on PS5 and would like to play with ps4. Which, to me is cross gen. BUT a lot of answers I've been getting are that it's cross gen progression rather than being able to drop into 64 player servers. 

Mods, heroes, computer says no's (I don't know how it works here) - bit of clarification?

Thanks

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