Quotas are applied to quota roots, which can be at any level of the mailbox hierarchy. Quota roots need not also be mailboxes.
Quotas on a quota root apply to the sum of the usage of any mailbox at that level and any sub-mailboxes of that level that are not underneath a quota root on a sub-hierarchy. This means that each mailbox is limited by at most one quota root.
For example, if the mailboxes
user/bovik user/bovik/list/imap user/bovik/list/info-cyrus user/bovik/saved user/bovik/todo
exist and the quota roots
user/bovik user/bovik/list user/bovik/saved
exist, then the quota root
user/bovik applies to the mailboxes
user/bovik/todo; the quota root
user/bovik/list applies to the mailboxes
user/bovik/list/info-cyrus; and the quota root
user/bovik/saved applies to the mailbox
Quota roots are created automatically when they are mentioned in the setquota command. Quota roots may not be deleted through the protocol, see Removing Quota Roots for instructions on how to delete them.