If you've got two domain. How can the UPN suffix help you in administration?

Adding these suffixes gives you the ability to use a friendly user-logon name that does not match the domain's or parent domains' naming structure. http://support.microsoft.com/kb/243629/ MCP/MCSA/MCTS/MCITP

Can you give an example to explain this for me?

UPN is the user principal name (user@domain), the UPN suffix is the "domain" part. Well, suppose you have a forest with multiple domains using their region or country as domain names. A domain would be city.region.country.company.com. Users would have troubles to remmeber this, and it might not always be clear to users whoch domain to choose from. Configuring a UPN suffix allows for selecting which UPN suffix a user must use to logon, for example "company.com" http://technet.microsoft.com/en-us/library/cc739093(WS.10).aspx MCP/MCSA/MCTS/MCITP