"Relays" constitutes a truly mind boggling realm. Within the realm there are even different meanings for the same aspect but in different power regimes or different areas of electricity.
So your question is too vague to really answer specifically.
In sort of a general sense, if a relay can be shoved up against a flat vertical surface and by the addition of screws or bolts can be mounted, then it is a "panel mounted relay". Many, many types have no such ability and so aren't.
Not too vague at all. Rack mount relays have "ears" for mounting on 19" racks. Panel mount relays have flanges around the relay to mount the relay into a hole cut in a panel and cover the gap around the relay. Rack mount relays can be panel mounted, but there will be a visible gap around the relay and the mounting hardware will be exposed. Panel mount relays to be installed in a rack need to be mounted on a filler plate and then the plate rack mounted.
