In the Bay Area they tend to be for AI companies, though I'm pretty sure I've seen at least one for an AI lawyer. (Meaning a general-practice lawyer that is actually a datacenter, not a human lawyer who practices AI law.)

It's actually kinda fun to look at the public transportation ads and see what they say about each major metro area. In the Bay Area most are for enterprise software vendors. In Boston most were employment ads with some obscure scientific or mathematical puzzle where if you solved it, it redirected to a biotech or software company's jobs page. In Baltimore, it was mostly for bail bonds.