VMPro and Advance Webhost Billing System (AWBS)

Separate web server. Not necessarily a separate physical server, though most of our customers do have many servers, and don't keep billing details on the same system that provides hosting service. Virtualized systems work, as well.

The reason for this is that Virtualmin sometimes needs to restart Apache. Since AWBS runs under Apache, and it loses state when Apache is restarted, it doesn't know whether a virtual server creation action worked when that happens. Virtualmin only restarts Apache under a few specific circumstances--SSL, new IP, module changes--but we'd still recommend running AWBS under an independent server unless you know you won't be using the billing system to perform actions that required a hard restart of Apache.

Virtualmin has its own web server, and so is able to do things that control panels that run under Apache can't do safely (like restart Apache!).