What is rel=amphtml?
- Use of rel=amphtml defines an AMP version of the current content.
- Defining the AMP page url allows Google to discover the AMP version of the current page.
The above code is basically saying...
- There is an AMP version of this page
- The version is located at amp-page.html
Where is it used?
Since rel=amphtml is defining where the AMP version of the page is, it must be located in the non AMP or standard version of the page.
The standard page will use rel=amphtml to point to the AMP version. The AMP version will point to the standard version using rel=canonical.
The main reason for rel=amphtml to exist is to discover and index AMP pages.
AMP and Google discovery
When Googlebot encounters a rel=amphtml it will follow and record the AMP page to eventually add it to the Google index.
Assuming the Amp page is valid it will be added to the Google CDN (content delivery network).
In the future when a mobile user searches for your page Google will use the copy from their CDN to display page to user.