It really doesn't
I mean, USUALLY it includes the "GFE" of BBBJ and kissing. Many would say that it includes anal and CIM or COF. However, there are others who argue it's just a style- GFE is more sensual/romantic, PSE is dirty/raunchy porn-esque sex.
Then,of course, there are actual porn stars who advertise a PSE, but don't do anal, CIM, etc- yet I think we can all agree it's certainly a porn star experience.
Recently, "PSE" has been used to advertise uncovered full service/BBFS in a couple of west coast cities. Nowhere else in the adult industry uses that standard, yet it's now common there.
The only (mostly) common denominator I've noticed, is that if there's an upcharge for PSE, then in those circumstances it usuallly includes anal. Still, in one of the cities where PSE is being used synonymously with bBFS, that might not apply and instead the upcharge is for uncovered services.
See? It honestly is all over the map. If you expect a common definition of PSE and use that to find a provider then you're very likely to end up disappointed.
Your best chance at figuring out what it means is simply reading a provider's TER profile and reviews, and perhaps backchanelling with previous reviewers.