All Sub Areas is a category on the Category Extensions boards for Super Mario Odyssey. The category involves collecting every moon inside of a sub area. The defined rules for a sub area are as follows:
"Sub areas are any area that will have Cappy say "I remember this place!" or "Doesn't seem like there's anything left to do here..." any time you enter it during the post game beyond the first time entering it."
There are a total of 161 sub area moons that must be obtained for a valid run.