On Shop Sheriff, we offer you the ability to choose a custom domain name to use for your AMP pages.  This allows you to use a subdomain such as "amp.yoursite.com" in order to host your AMP pages.

We have found that users like the custom domain option because they like the simple url structure of subdomain for their AMP pages. Most commonly users chose the subdomain "amp". Other users like something such as "m.yoursite.com".  There's no difference in Google's eyes.

Using a custom domain is not necessary, and there is no SEO benefit or penalty if you do or do not use it, but we allow a user the option to choose this purely for convenience.  Some users prefer having their AMP pages all on a separate subdomain.  If you aren't sure about which one to use, then the Standard Configuration is great!  Both the Standard and Custom Domain options offer the same SEO benefit, and one is not better than the other.