I actually just invented the term "MCI" because it didn't exist yet (well, it existed, but there was no real term for it as far as I know xD).
MCI (Multi-Cast Instanceable) means that every unit on the map can cast the spell a lot at the same time without it getting bugged (so 1 unit can cast the spell 2+ times).
MUI (Multi-Unit Instanceable) literally means that every unit on the map can cast the spell once, once the spell is done he can cast it again (when a unit tries to cast it twice, it would get bugged if it's only MUI).
Most people use "MUI" for "MCI", but I think it's different... just like there's a difference between MPI and MUI