My personal rundown and how they get assigned:
E - Educational / Lower office personnel spec
L - Office personnel you hate spec, but don't offer the E because they might complain.
T - Give this to all the technicians because they can't take care of anything and it will survive typically.
P - Give this to the engineers who believe having an RTX gpu will actually help them so that they are happy, and to the CAD operators who actually need it.
X - Smaller/Ultrabooks before the term got started, now somewhat a blurry line because T series have gotten lighter/thinner. But the X1 Carbon sure is a great way to spend a ton of money for a light laptop when a T-series would suffice.
Personally I stick to older used X series (currently x250) because I just enjoy a small laptop and they are dirt cheap now.