What is the thirtieth Ugaritic sign, S2, actually for?

The sign appears at the tail of the longer Ugaritic school abecedaries, written a second time after the twenty-ninth sign, and its shape is conventionally described as samekh-like. No Ugaritic source names it, and specialists disagree on its basic function: some read it as an additional native sibilant the ordinary twenty-nine signs did not distinguish, others treat it chiefly as a device for spelling words of foreign origin. No other script modelled in this atlas offers even an approximate cognate to compare it against, unlike Ugaritic's other five signs unique among its Northwest Semitic peers, all five of which correspond cleanly to a preserved Arabic letter.