Boot partition first. Then any other partition. Not any different from the logic by which the workbench and icon libraries are loaded. Actually, since A4000T times.
On A4000T it was already quick&dirty work-around, and back then it was not an issue as those two components either were in kickstart, or they weren't, in which case they could be loaded from disk.
The case now is different, we are now not just talking about components that may or may not be in kickstart, but about updating just about any components that are in kickstart, with rather randomly available components existing on available file systems.
So if you boot from floppy on a system that has a hard drive with several boot partitions that have different versions of workbench.library and icon.library - what can one expect?
Does it check other bootable filesystems first, using boot priority?
Does it just pick the first version it finds that is an "update", or does it continue to scan and pick the one with highest version string?
Does it care about pure flags, like LoadModule now does?
Does it do any validation, or will it blow up if it "by accident" finds non-native modules from available on AROS, OS4 or MorphOS partitions?