I don't think there are any hard and fast rules, but maybe expectations?
GFE involves kissing and snuggling and maybe, I don't know. I guess it is sweeter? More intimate? Personal? That's how I think of it. It feels real. It has real elements to it. I heard one provider say she allowed herself to fall in love for an hour. Kind of like that. After a while you learn how to turn it on and off - but it is not easy. Compartmentalizing is never easy.
Almost GFE would involve most of what you expect with GFE but it woud be missing one critical element, like it would be missing kissing, or it would exclude oral on her, or something like that, depending on her preference. Almost GFE is one of those nebulous gray areas. I think it is different for everyone. I knew a popular provider who did not kiss but I would say she was a GFE in every other respect.
When I asked my hobbyist ex-BF who had seen LOTs and LOTs of providers what was the diff, he said non-GFE tended to involve less talking, less foreplay, less or no kissing, everything straightforward and on the clock. Nothing wrong with that if that's what you want that day, right?
I guess role play and wardrobe and acting for a kind of specific scene, would not nec. call that a GFE experience. I have done that and I don't know WHAT you call that! 
Fun and games, maybe. Variety.
And then when I asked him what was PSE? Does it mean you have to have been an actual porn star to be PSE? He said some think of it that way, but some don't. And he said it was vocal - but then he said you're GFE and vocal and so I said well, what is it then? And he said well, like what you would see in a porn video. But that is so broad! You see everything in porn! You see GFE in porn! But I guess it was like -- you know, kind of wild and crazy, I'm ovulating, f-me encounter. I don't know.That's how I think of it. What do I know? I think everybody has a different definition.
For Greek, I don't think that is covered by the GFE, non-GFE or PSE umbrella. I think Greek is YMMV. There are GFEs that do it and some who don't and some who do it sometimes...so that is a YMMV unless you know upfront that is on the menu.
And that's the trick, right, because technically it can be a bad idea to say what's on the menu (or ask) and just makes it all complicated but kind of fun too, because you're not really sure what is behind Door A or B
