What you see is based on the following:
A: 4.0 W/kg FTP or higher
B: 3.2 W/kg to 4.0 W/kg FTP
C: 2.5 W/kg to 3.2 W/kg FTP
D: Under 2.5 W/kg FTP
When you join an event or race, you self-select. If it is a race and you exceed the high limit of your category, depending on the race you may or may not be disqualified from the results.