It seems a miscut is when part of the picture/border/etc. is 'missing.' If something is 99.9% off centered, but nothing is actually missing, it gets the OC. If a hair of the picture or actual border (not the 'white areas') is missing, it gets the MC.
In my 1969 WL Mantle in this thread, a tiny bit of the round number bubble on back is cut off, so it was deemed MC.
Edited to add PSA's definition:
MCdefinition.jpg