To retire a business unit, click on the name of the Business Unit and select Retire.
If you have retired a business unit, you are still able to reverse this by clicking on Unretire. To find your retired business units, click on Show Retired on top of your business unit tree and then Refresh. The Retired field will be shown with a circle icon next to it.
Please note that if you have users against a business unit, you will not be able to retire the business unit until you have edited the users and changed their business unit.
