Modern breakers have adjustable or interchangeable trip units, to allow the trip settings to be adjusted to meet the required protective levels.
Your "100AT" designates that the circuit breaker should be configured with a 100 Amp trip unit, or perhaps a trip unit that can be set at 100 Amps.
Depending on the details of the breakers you are using, this 100 Amp Trip breaker will require a frame size of 100 Amps minimum. So, you might see 100AF/100AT, or 250AF/100AT, for example.
The frame size is the designation of the maximum continuous current the breaker can carry, and also an indication of its physical size. A 100 AF breaker will be smaller than a 400 AF breaker.
Frame sizes are determined by what the manufacturer offers in their breaker catalog. Typical values for molded case circuit breaker (MCCB) frame sizes are 100, 250, 400, 600, and 800. Above this size, they go to low voltage power circuit breakers (LVPCB) which are much larger and more expensive.