Classifications
Within the UK, Elite and Cat 1+2 riders generally race together and Cat 3+4+W separately; however individual events will vary.
Within the USA, the Men's Field (Pro + Cat 1 + Cat 2, and sometimes Cat 3) generally race together, Cat 3's often have their own races, and Cat 4/5 Men race together; however, certain states (e.g. Colorado) do not offer Cat 5 due to a limited number of participants. so Cat 4's will have their own races. In addition, there are a variety of Masters categories which can be raced. The Women typically have two separate races, the P/1/2 (3) and the 3/4.
Collegiate racing in the USA is sanctioned by USA Cycling (UCSF) and consists of three categories: A, B, and C. Category A is equivalent to the P-1-2 field, category B is equivalent to the Cat 3-4 field, and category C is equivalent to Cat 4/5. Thus, collegiate criteriums are organized accordingly.
The races will also vary depending upon how many people from separate teams enter, which will impact whether it will be a "free-for-all" or a team-focused event.
Read more about this topic: Criterium