How to set up a custom domain prefix for my website's url
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I just need to know what the name for the mail in:

**mail.**google.com

would be named

I thought 'custom domain prefix' made sense, but that doesn't seem to be it.
**Please provide a link to documentation if possible, or just a proper search query.**

Also, would it be a good idea to use these prefixes? Basically I have a site called smokelessmi.org and I want to make a prefix a2.smokelessmi.org for the city (Ann Arbor, Michigan) since this is a localized website.

Eventually I want to add other cities in Michigan, but I'm just starting with this one.