What is the difference?
I mildly understand that Candidate Master is a 2000-2100 FIDE Title, but I've seen several USCF players with that under their "Norms" section, in conjunction with 1st Category, 2nd Category, etc.
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The USCF has two independent systems for naming players' levels:
1) Rating-based: Senior Master (2400+), National Master (2200-2399), Expert (2000-2199), Class A (1800-1999), Class B (1600-1799), etc. This describes your current rating; you can be Class A today and Class B next week. The one exception is that once your rating gets to 2200, you are a master forever. These names and rules have been around for decades and are what most players are familiar with.
2) Norm-based: Life Senior Master (2400+), Life Master (2200+), Candidate Master (2000+), 1st Category (1800+), 2nd Category (1600+), etc. You are granted one of these titles by achieving five norms of that level and getting to the minimum required rating (at least once). Generally speaking, a title norm is a tournament performance that would be impressive for someone who had the minimum rating for that tile. These norms are what you see when you look at a USCF player's rating page. The full rules for norm-based titles are available online. One intent of norm-based titles is to provide an incentive for people to play in which they can't "lose ground", like they can with their rating. These have been introduced more recently and many players are not even aware of them.
According to Wikipedia, the USCF title of Expert is awarded to players rated from 2000 to 2199 (USCF rating!). The title is not for life.
So, the two titles depend on two different ratings. The question that now arises is: how can we compare the FIDE rating with the USCF rating?
According to the graph of this article, a 2100 USCF rating roughly corresponds to a 2000 FIDE rating. So it seems that, usually, a FIDE-CM is a stronger player than a USCF-Expert.