In the acotr editor, if you select a piec e of equipment higher than 0xFF (256), "unarmed" (0xFF) will be selected.Plus the data is still on 1 byte so it's impossible to go higher than FF. This is because the advance version has more items but I highly doubt that the extra items have their own graphics.
In the monster editor, if you select a normal attack looks higher than 0xFF (256), "unarmed" (0xFF) will be selected.So if by any change you change that data manually, changes will not be reflected in the editor. In the monster editor, the only hardcoded data are the items names (4) of the morph patters.I simply took the information mainly from FF3usME and applied it to the advance version. Since the data is the same, I'm assuming they mean the same thing as in FF3us. All the bytes properties in the monster, item, and spell editor have not been tested nor inquiries made on them.So far only the monster data, item data, spell data, actor data and fonts can be edited. I've started making an editor for FFVI Advance on GBA.