I mean whatever is needed for the community to replace/maintain the servers, just as I said.
That would obviously include the network protocols, but might also include data structures, API contracts, map data, timetables, and any number of other things.
I wrote in general terms deliberately, since it would mean different things for different games, and to allow for the possibility of releasing source code instead of descriptive specs.
(And no, source code is not the only way to do it. If that were the case, the community-developed game servers that have been made through reverse engineering could never have existed.)