When an automatic alias domain is created for a new virtual server, this option determines if the first part of the alias domain is the complete domain name of the virtual server, or just the part before the first dot. For example, if the domain to create under is `isp.com` and the new domain is `foo.com`, selecting Entire domain name would cause the alias to be foo.com.isp.com, while selecting First part of domain would cause it to be foo.isp.com.

Alternately, you can select Virtualmin template and enter an arbitrary expression that makes use of template variables like ${DOM}, ${USER} or ${PREFIX}. Randomly generated prefixes up to 9 digits long can be created with a variable like ${RANDOM5}, which creates a 5-digit random number.