RBM wrote:

"Main breaker" is defining it's usage, not necessarily how it's built. The
same thing with a breaker used as a service disconnect. The breaker is not
necessarily different from a similar type breaker used in the same panel,
however when used as a service disconnect, it requires a hold down method

Another difference between the main and regular breakers is that on the
main, the power is fed from the source to the terminal lugs, then to the
busbar. A regular breaker gets the power in from the busbar and out
through the terminal(s). Can breakers handle power feeds in either
direction?