Learn more about specifying domains in a certificate
This page is basically a worksheet that helps you to determine the internal and external host names that are required in the request for a subject alternative name (SAN) certificate. If you enter a value for each service based on the location (internal or external), the wizard determines the host names that are required in the certificate, and the information is displayed on the next page. To modify a value for a service, click Edit () and enter the host name value that you want to use (or delete the value).
If you've already determined the host name values that you need in the certificate, you don't need to fill out the information on this page. Instead, click Next to manually enter the host names on the next page.
For more information, see Certificate requirements for Exchange services.