alternative global variable to siteUrl or the primary website?
You can define whatever aliases/environment variables you like. In this case, I'd probably define an environment variable in my .env file like this:

BASE_WEB=http://example.com


And then in config/general.php I'd add an alias:

&lt;?php
/**
 * General Configuration
 *
 * All of your system's general configuration settings go in here. You can see a
 * list of the available settings in vendor/craftcms/cms/src/config/GeneralConfig.php.
 *
 * @see craft\config\GeneralConfig
 */

return [
    // Craft config settings from .env variables
    'aliases' =&gt; [
        '@baseWeb' =&gt; getenv('BASE_WEB'),
    ],
];



And then to use this in a template, you'd just do:

{{ alias('@baseWeb/assets/css/main.css') }}


...and away you go. To learn more, check out the Flat Multi-Environment Config for Craft CMS 3 article.