Alias URLs - Enquiry

In the process of setting up my tracking regime I ran across an advanced option called Enable Cross Domain Linking in the Websites/Tracking Code/Advanced panel. One of the requirements is a list of alias URLs that are entered in the Websites/Manage/URLs panel. In this panel I found the following URL

Now, this hardly appears to be an alias for the site that I am seeking to track – namely, Rather, is the domain in which the matomo software is located.

Without being overly repetitive, please recall that and are add-on domains that occupy separate folders in a parent folder that represents the principal domain name of my host server account.

Are and being treated as subdomains of – the parent folder?

What exactly is an alias domain in this context?

Could someone please clarify what is going on.