Way back when, no gal would ever kiss, even if you could do just about anything with her (I'm talking about the time of the land without AIDS, when BB was very common.)
Then, the term GFE came along, and its most defining differentiation concerned kissing. So with that in mind, here's my take on the gal's tiers of service:
Slight: A peck on the lips, mouth closed.
Partial: Mouth open, but no or very minimal tongue. (Tongue tips can touch, but no more.)
Full: Tonsil hockey.
Tiers of service is not uncommon, by the way.
One gal I see has three as follows:
Spa session: Massage (very good one, by the way) with a happy ending.
Encounter: Massage with BBBJ (no completion) with a body slide to complete.
Relationship: Massage with covered FS ending or BBBJTCNQNS.
Since the second tier is only a bit more expensive than the first, I opt for that. The third tier is quite a bit more $. She offers DF kissing (not DFK), with all three as part of the "get warmed up" make out session.
Many gals offer two tiers with FS being the main difference. Some allow BBNQNS, others NQ, and some no completion. A few allow COF however. Strange thing that. Some gals will drink all the cum you can offer, but freak out at the suggestion of cum on any part of their body, let alone their face. On the other hand, one gal won't allow me to cum in her mouth, but insists I finish on her face. Go figure.
And of course there's the old GFE/PS duality, where greek is the defining factor, generally speaking.
There's a lot of variation out there, which is all a part of the fun.