When adding new domains (and adding it to a hosting package / website) for a specific customer..

I wonder where a total Domain list is listed?   Also; when I detacha domain from a website (of a hosting package for a specific customer), where is that domain name now..?   It is still in the Windows DNS (where it was when I added the domain name).

As an admin I would like to see a total domain listing (with active/inactive domains)..  I am probably looking at the wrong pages but I really searched more than an hour.