A: from our new AppSumo faq:

Each administrator account is siloed into it's own environment, and can create customer accounts and manage email delivery connections completely isolated from all other administrators.

This means with a Tier 3 license, it's like running multiple installations of the platform on multiple servers simultaneously, even though it's actually only running on one server. Each administrator account is like giving you another virtual ESP.

Licenses are only valid on one server at a time, if you need to move servers, the license will be revoked for the first server when you use it on the second server.

https://docs.emaildelivery.com/docs/faq/appsumo-select-faq

Administrator accounts are like a cheat code basically, they let you bypass needing more servers.