The mailboxes that have been disabled, but which still remain on the servers for a period of time as defined by the mailbox ...

The mailboxes that have been disabled, but which still remain on the servers for a period of time as defined by the mailbox retention policy--typically 30 days. If the disabled mailbox can be traced back to a previous SetMailboxType value (such as MBX=DW;), then it will be counted in the metric for that mailbox type (such as "Mailbox - 500 MB - Deskless Worker"). Any remaining disabled mailboxes that cannot be traced back to previous SetMailboxType values (present before the mailbox was disabled) will appear in this metric. Disabled mailboxes are treated for billing purposes as Standard mailboxes.