These are some notes from Schola San Marco’s Fiore Seminar in November 2013.
The crown represents the master who is introducing the start of a section.
The garter is used to indicate who is acting in the play.
Most garter plays refer back to the previous crown, not the previous garter.
Usually, but not always, the garter is on the leg that will be moving next. Note: that leg may not necessarily be moving forward.
I can’t say that I am convinced of the moving leg thing.
Crown and Garter
If you see both a crown and a garter then it represents the beginning of a series of counters to the last crown play.
Like in comic books, there are indicators of motion. This can be seen by the clothing trailing behind when the actor quickly moves forward.