Hi
if I'm not wrong optionals are processed in this order: tce, cde & iso.
I think this is philosophically wrong.
tce is the main extension dir and it's also the one who it's writable so could conain more updated extensions then the ones in cde or iso so it's fine to being processed first.
I think iso should be processed before cde cause it'is easier to modify an iso file rather then go to the shop, buy a newer cd and burn it, iso normally contains more uptodate extensions compared to the cde ones, but if they are processed after the cde ones they won't overwrite older extensions during boot.