You got it correctly, it test if a segment is completed if you cross the normal line to the path at the final point. This is because you don't want to go back to your point while doing a segment or a path, even if you you miss the final point for a few meters. This is especially true for fixedwings, but I think it also applies to rotorcraft. The test is only done if the initial waypoint ID is valid (>0 and <NB_WAYPOINT).