I don't know about containers other than mkv, but according to the man page for mkvmerge, which track is selected is chosen as follows:
1. If there is a default track, that track is selected.
2. If there is no default track, then the first track is selected.
3. If all tracks have their default flag set to false, then no track is selected.
I don't know if it's possible for multiple tracks of the same type to have their default flag set to true, but I would assume that in such a case, the first track with its default flag set to true would be the one selected.
Personally, what I care about is the subtitle tracks, but I believe that the aforementioned rules apply to all types of tracks. MPlayer follows these rules. SMPlayer does not.
In the subtitle preferences for SMPlayer, you can select whether the first available subtitle is selected. If that checkbox is checked, then the first subtitle track is selected regardless of what the default may be. If that checkbox is not selected, then no subtitle track is selected regardless of what the default may be.
I can understand SMPlayer having additional control over the subtitles (thus being able to force which subtitle track is selected), but I would like to be able to have SMPlayer follow the rules for mkv tracks correctly - which MPlayer appears to do just fine on its own. I don't know what the rules are for other containers, but I would hope that if there are no overriding preferences in SMPlayer, that they're rules would be followed as well. In either case, I would very much like SMPlayer to be able to handle mkv tracks correctly like MPlayer does.