Multisite - subdomain + subdirectories

I would like to have some opinion from members, do sub-domain-multi-site Installation can create sub-directories ?

example of selection while do the multisite installation

subdomain : aa.domain.com bb.domain.com
subdirectory: domain.com/aa domain.com/bb

CAN OR NOT? login via aa.domain and create subdirectories to be aaa.domain.com/123

Currently I do separate installation on various subdomain.

Sorry for broken English.

Thank you.