Which riders are the race
Zwift gives every category of an event its own subgroup, and a rider only
ever carries their own. That's the right field for a category start
— A goes off, then B, and you're racing the riders in your pen. It's the
wrong one for a mass start, where every category leaves on the same
gun and rides the same road: screening to your own category there deletes
most of the riders you can see.