Three critieria come to mind to distinguish an Ultima variant from a Chess
variant with some Ultima elements (this might also be a useful thing to
index):
1. More capturing move types than non-capturing move types.
2. Majority of capturing move types are non-replacement.
3. At least one piece with an important special power (e.g.
Immobilizer).
#3 is not a strict criteria but is indicative in borderline cases.
Chess On A Longer Board . . . is clearly a Chess variant with Ultima
elements by these criteria.