Every Exchange Online tenant has four mailbox plans. Exchange uses the plans to populate some important mailbox settings based on the license assigned to the mailbox owner. This article explains the four mailbox plans, how to update the plan settings, and some of the things you can’t do with mailbox plans. We also include some PowerShell to report the mailbox plans assigned to users in your Office 365 tenant.
https://office365itpros.com/2021/10/26/how-exchange-online-uses-mailbox-plans/
