Close but not quite correct. Zscript is unrelated here.
If you'll open SIGIL.WAD you find the following:
MAPINFO and short SKY5_ZD texture for ZDoom v1 family (ZDoom v1, ZDaemon, Skulltag) and some ZDoom-unrelated port which support this feature too;
ZMAPINFO and GAMEINFO for ZDoom v2 family (ZDoom v2, GZDoom, Zandronum, QZDoom);
EMAPINFO and EMENUS for Eternity engine (not sure if there are derivatives).
Effectively, yes -- it is mostly ZDoom-targeted WAD. But selection is strange.
On ther other hand SIIGIL_COMPAT.WAD containes only one port-specific feature: embeded DEHACKED (actually BEX) patch. Therefore it can be used with almost every limit-removing port, including BOOM-family (BOOM, MBF, PrBoom), Chocolate family (but not the Chocolate itself), less popular ports like EDGE, Vavoom, Doomsday, Risen3d and even with properly hacked vanilla engine.
Must admit that handful of 'unpopular' ports are perfectly capable of handling additional episode and menus. Some of them even had this capability long before ZDoom came to popularity. They just didn't got attention from the 'King' and dropped to the category 'everything else'.

Sorry, my bad. No idea what made me type Zscript and I had looked through the WAD just a few days ago and yet somehow completely forgotten that the non-ZDoom lumps were there as well. Still the general point is the COMPAT files are made for (DeHackEd/BEX-supporting) ports/hacks which are not compatible with the WADs' methods for defining additional episodes, but the COMPAT files are not actually (non-hacked) vanilla-compatible.


While Chocolate Doom supports DEH, it is not limit-removing, so SIGIL_COMPAT.WAD does not work on it. (Crispy Doom is both limit removing and supports BEX though).

Yes. Figured this out and even fixed my post. But somehow missed you reply, so now that post looks strange. Sorry.
Chocolate bombs out with visplane overflow just like the vanilla. What actually is a feature, accordinly to thr offical FAQ. )))

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BTW, yet another one story about running Sigil in MS-DOS on a real 486. With an assistance of BOOM source-port, though.