Fill
What fills the frame where no clip's pixels land.
Fit.Contain can leave bars, a track that starts after the composition does leaves a gap before it plays, and rounding can leave a frame a pixel short of an edge. This is what fills all three.
More arms will arrive as filmstrip adds fill kinds, so a consumer must handle one it does not recognise rather than assume the set is closed. filmstrip's own backends match every arm, so a new one does not build until each of them lowers it.