The letters "u" "CU" and "PU" show on thumbnails now. What do they mean?

P is Parent post

C is Child post

U is probably Unapproved

Parent, Child, Unapproved, Flagged.

Genjar said:
Parent, Child, Unapproved, Flagged.

Good to know. I've been wondering about this too since noticing them the other day after the usual E, Q for questionable or explicit.

HasParent, HasChild, Unapproved, and Flagged. This has a C despite it being the parent post, and This has a P despite it being a child post, as evidenced by this screencap.