Quotas
Usually
users will only be associated with a single profile, but it is possible for the admin user to connect a user with multiple profiles.
All
profiles are controlled by what
quota they are in. A profile can only ever be in one quota. The admin user can change which quota that profile is in, but it can never be in 2 quotas at the same time.
EG: The admin user might want to change a profile from one quota to another quota when a subscription fee is paid.