I'm looking on DL.com at flights from CHS-LAS on 1/4/2023, and I'm curious about something I don't recall seeing before (granted, I've been flying UA mostly since 2019).
Several flights are showing "Not Offered" for first class. When I look at the seatmap for those flights I see that almost half of First and half of Comfort seats are allocated but the rest are blocked. What is there to know about this?

(Despite using site search & Google site search, couldn't find anything... apologies if this has been asked & answered before.)