Not a lot of point in distinguishing between STOBAR and CATOBAR in my opinion... In all likelihood CATOBAR jets can operate on STOBAR carriers, and the possible weight reduction advantage by not having the launch bar and some other equipments are offset by the time and effort taken to redesign these aspects, as well as the fact that most likely there will only be 2 STOBAR carriers for the navy, meaning a very limited number of these STOBAR aircrafts needed.
Hence what I assume to be the revision from 4 variants to 3 with presumably only 1 naval variant doing double duty as STOBAR and CATOBAR.
But ofc that's just pure speculation on my part as to what the 4th
might have possibly been when taking into account the considerations at the time of FC-31's development during late 2000s/early 2010s, when CV-16 was just finishing reconstruction, and 17 had just been approved and wasn't even laid down yet. So we're talking about an eventual CATOBAR carrier that was at least 15 years away at the time, which turns out to be the case.
Now I don't know what timeline SAC had projected for the J-XY back then and how early in the conversation was the PLAN on the aircraft's potential procurement. Did they think it would take another 12-13 years to finalise a naval variant, CATOBAR or otherwise, for a CATOBAR carrier that won't come for another 15 years, meanwhile the PLAN already has 1 STOBAR carrier nearing completion and another one on the way? Just something to consider.
Still, while having 2 carrier wings of J-35 of a bespoken design to STOBAR standards may not sound like a lot, 24-36 initial airframes and maybe up to 60 or more in the years to come as the J-15s slated for 16 and 17 begin to retire isn't exactly non-trivial either, which I think a case
could be made for a STOBAR-optimised J-35 and the value such a variant could bring to these 2 carriers, chief amongst which, of course, the weight savings from these optimised airframes thus affording better range/endurance, larger payload etc., especially as the boats remain in service with decades more yet to go, and the associated air wings along with them.