I typically see content flagged NSFL when it's generally repulsive regardless of background (excepting those seeking out the content). Stuff like graphic videos of beheadings or people set on fire that's upsetting even to people with no traumatic background. It's kind of like a trigger warning for an average person; background doesn't matter if the content is bad enough.
NSFW/NSFL is a completely different animal than trigger warnings. Only 3 categories exist: 1) no warning, 2) NSFW, 3) NSFL.
Trigger warnings are hyper-specific to the audience and could involved thousands (or infinite) potential 'triggers' and a huge variety of audience categories/groups. The burden on the platforms, writers, and general audience is magnitudes higher for some questionable value.
