When I create a new user in exchange, it asks me to provide the User Logon Name. There is a dropdown box that supposedly allows you to select a domain for the user's login. What is this referring to? How can I make it so that I can create users with different domains in their user logon name?

p.s. I am very aware of 'Accepted Domains' in Exchange allowing one user to have several email addresses in different domains. I am just curious how I can modify the user's Logon name specifically.