The San Francisco 49ers host the St. Louis Rams on Sunday to close out the 2015 regular season, and the game is going to air on CBS instead of FOX. This slipped under the radar to some extent, but when the league formally flexed Packers-Vikings into Sunday Night Football, they also moved 49ers-Rams to CBS, and Raiders-Chiefs to FOX. Both games kick off at 1:25 p.m. PT.
Normally, FOX hosts NFC games (and inter-conference games hosted by AFC teams), while CBS hosts AFC games (and inter-conference games hosted by NFC teams). However, the league recently expanded to flexing option to allow games to be flexed between networks. It does not happen all that often, but it is essentially a way for the league to even out the competitive matchups on each network.
It would seem odd to swap the 49ers and Raiders games, but Matt Maiocco provided some helpful reasoning on why it might have happened. FOX lost a prime game when Packers-Vikings for the NFC North title was moved to NBC's Sunday Night Football. The Chiefs can still win the AFC West title, and I imagine the Raiders would enjoy preventing that. 49ers-Rams could end up a decent game, but I can't imagine ratings will be particularly good for the game. Heck, I barely want to watch it!