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.