Contactor vs. Breaker-type Automatic Transfer Switches
Thanks for the link. I edited my original post because it is now clear what you are looking for.

EDIT:
The circuit breaker type uses actual circuit breakers to do the transfer switching. This offers additional overload protection. A simple contactor type of transfer only switches the load from each source of power, not offering any over current protection. Additional protection must be in place upstream of the switch. 

In either type a load study should be done to ensure proper load shedding, which includes upstream protection for either type. You did not offer any insight as to what this is for, but if it is for industrial applications then it is common practice for a PE Engineer to approve such a load study before the job is contracted out.