Up to Freeciv 2.4 the classic ruleset was known as default ruleset. Starting with Freeciv 2.5 the …/data/default.serv consisted of a rulesetdir classic server command. For Freeciv 3.x civ2civ3 is expected to be the new default. As of 2.5 there are already separate …/data/classic.serv for rulesetdir classic and …/data/civ2civ3.serv for rulesetdir civ2civ3.
The initially loaded ruleset, as defined in a …/data/default.serv or set by command-line option -r, inherits any saved options. Actually option -r only tries to read a file with server commands, this does not necessarily include a rulesetdir command. Likewise clients present found foobar.serv files as foobar ruleset, instead of, say, enumerating all sub-folders containing a game.ruleset file.