They're different things. The briefing triggers the corresponding MissionStart event. So, if you play a briefing that says: "Mission 1 3", it'll call the MissionStart event of the episode_X.rules with the ActEpisode parameter evaluated in 1 and the ActMission in 3. From there, what mission is played is up to the contents of the MissionEnter rule (with the same parametres as above).
Bottom line the MissionPlayBriefing event is played before the MissionStart event, and not only that, but the briefing actually calls that event. So the cronology would be something like this:
Briefing plays and it calls the MissionStart based on it's "Mission X Y" line -> the MissionStart then calls (after the actual mission) the MissionPlayBriefing based on the uSetNextMission(X,Y); command -> if there's a briefing, it'll be played and the cicle continues, otherwise the MissionStart event is called with those parametres.
So I'd have to answer that, yeah, if the briefing for 11 has played out odds are that the mission 11 will be played, unless you do a pretty shaddy work with conditionals and ifs in the MissionStart and MissionEnter event.