Object XYZ contains other objects. Are you sure you want to delete object XYZ and all of the objects it contains.

This is just a warning for you to double-check if the objects associate with XYZ can be removed too.

 

To view the associated objects, select
ADUC - View -- Users, Contacts, Groups, and Computers as Containers
Now expand the object and check all associated objects.

Once confirmed, they can be removed, tick - Use Delete Subtree Server Control and then Yes to delete the objects.