In Revolution, site_url is the setting you want, and this can be overridden (and hardcoded if you want) by Context. Otherwise, it is automatically determined based on the host name requested by the client and/or the web server/php configuration.
[[!++site_url]] <!-- This returns the site_url setting and the ! indicates this is not cacheable -->
/* gets the site_url setting */
$site_url = $modx->getOption('site_url');
Generally, the only place you ever need this is in the HEAD of your MODx Template( s ), e.g.:
<base href="[[!++site_url]]" />
That way you can use relative paths in all your content and it will always respect the absolute location defined by your site_url, regardless of where/how it is deployed.