I first thought that "kiu/tiu" is strictly about people and "kio/tio" strictly about things like my native language makes that difference. Then I learned "kiu/tiu" is who and "kiu/tiu + substantivo" is which. Once I grasped that, I thought this is easy, until… I learned that while there are plural and accusative forms of -u and -a, there are not for -o. That is something I find strange and alogical.

'Kioj' is not prohibited, but is rare uzed. Set ('aro') of things is one thing, so they commonly use 'kio' in this case. But for set of sets they use 'kioj'.