The tab "Address/Contacts" lists all created addresses of your client. If you right click on the entry a dialog will open.
Here you can:
•Easily duplicate the contact
•Call the client if you've purchased the cti module
•Send an email to your client
•Open the website of your client
•or copy the address to the clipboard to use it in another program.
The "Main contact" is used for the presetting of the email forms in Xpert-Timer. So if you send a report through email, the main contact will be pulled from the client info, unless you've entered a specific other contact person in your project settings.