My circuit breaker panel cabinet has 2 existing, already-installed
breakers with nothing attached. I would like to use one as a
dedicated line for a spare refrigerator in the basement. All of the
existing bx-cables exit the cabinet from the top, and _every_ existing
panel knock-out on the top of the cabinet is occupied. There are
knock-outs on the side of the cabinet, but I’m wondering, is there
some rule or code that prohibits exiting from the side?

All advice appreciated.
Thanks.
-Theodore.