The sequence number is used to identify a track numerically. This is mostly applicable to type 2 MIDI files, however, it can be used for other types should those files be intended to be part of a related collection.

This is stated in the specification as intended to allow a sequence to be started/triggered via a Cue (FF 07) event. However this cue event data is textual, so unfortunately, it is up to the implementation to specify the nature of conversion or format of the cue data.

I know this question is old however, I have come across it a few times and see it never had a clear answer.