this entire page:
https://imgur.com/a/6AgHMV4
is literally mxy explaining the CURRENT--as per snyder/morrison--multiversal structure of dc:
"BEHOLD THE MULITIVERSE....IT IS EVERYTHING EXISTING AT ONCE"
the 6th dimension is "the very tip top" of everything. it exists BEYOND IMAGINATION, but it stills exists within the multiverse as defined by the source wall. it's also where the wf and his brothers go to reform when they die. how could they reform OUTSIDE the source wall...? 😑
it also clearly states the 6th dimension is BEYOND imagination, meaning the 5th dimension is below the 6th. again, imo, the clear implication is that all these dimensions reside withIN the multiverse as defined by the wall.
https://imgur.com/a/6AgHMV4
the source wall was the shell holding the multiverse in place.
that scan that says the 5th dimension is outside the multiverse is similar to the macroverse business phil was discussing with op (and i agree with phil, btw): new info supersedes older info. in light of synder's work on metal and the jl (all based on exploration and further development of morrison's map) that scan has either been retconned or it needs to be re-interpreted.
if the 5th dimension IS outside the multiverse (ie--passes beyond/through the source wall), that means it exists in the greater omniverse, (or in the overvoid, but i think the overvoid needs to now be backed up to surround the OMNIVERSE, which....leads to some cartographical problems in itself....)
that makes no sense in the context of the story imo, nor does it fit with mxy's OWN description. i do NOT, however, think that scan needs to be 'retconned'. it can still be right--the orrery of worlds is certainly a multiverse in itself. the 5th dimension exists beyond the orrery, so it can easily and correctly be said to exist outside the multiverse--just not outside the...greater mutliverse, or source wall, which, again, makes no sense.
there is also a scan in one of the league books that talks about the how if you travel far enough in any universe/dimension/world you will eventually reach the source wall. maybe someone with a better memory remembers where it was and can post it. 😮
in any event, based on the story and the the description on morrison's map (here is the limit to even thought. beyond lies only monitor mind, the source and the unknowable) i don't think it can be anymore clear that the source wall=the boundary of all dimensions/universes within the redefined dc multiverse. /shrug