I used to be a screener at SeaTac.

The first class lane at my checkpoint led to the TSA person who inspected your boarding pass and id.

AFTER that the first class lane vanished and became whatever kind of lane it happened to be, closed, ordinary, mostly for airline or airport employees....it could be anything. I often worked a lane that started out looking like first class, but after ID check became SSSS screening. Yes, I was often screeched at by first class passengers for that lane being a lousy slow first class screening lane. It wasn't, it was a lousy slow SSSS screening lane that they had assumed was a first class quick lane.

We were told that the airlines established where first class lanes were, then TSA ID checks, then TSA established what kind of lanes existed after that point, where and when.