Both primary and secondary admin roles are the same except that the primary admin may invite other administrators. The secondary admin cannot invite another admin to join. Assigning or inviting a role does not remove the role from the assigner.
For instance: When starting service, a primary admin is setup for all the congregation that subscribe. The primary admin will thereafter likely want to invite a primary admin to each congregation so they can setup their own attendants emails for count, passwords, etc.
The new admin for a congregation may also wish to invite a helper as a secondary admin. A primary admin or secondary admin can view/edit the specific congregation where they have been invited to that role.
If the brother administrating is leaving or no longer wants the role of a the primary admin, he can invite another brother and then his role can be deleted.