When the system is started, it automatically detects the fully-qualified hostname of the system using data found in the SYS:/ETC/HOSTS file and DNS.

The $m macro is used to store the domain part of the fully-qualified hostname. The system will look at the fully-qualified hostname and store any information found after the first "." as the local domain name. If the local system is on a sub-domain (such as host.subdomain.domain.com), then everything after the hostname will get stored in the $m macro (subdomain.domain.com).

This data is built automatically by the system, and is not represented by a string or an expression. This macro is blank on purpose.

If you need to force the system to appear to be on a separate domain, you may need to manually define the $m macro with that domain name.