Master Pages from existing pages

I am using an existing website template, its setup as a project in Pinegrow. I need a better understanding on how to create a master page from an existing page.

When I go to Actions for Page I have the option to select a page as a Master Page. This I understand.
The part that is not clear is that it states you have to click on an element on the page before you can create a master page. This page is already completed as well as 18 other pages that need to be tied to the master page.

The question I have is related to having to click on the element prior to creating a master page. Does it only recognize that element on that entire page? Or does it recognize the entire page as a master page regardless of the element you clicked on.

It would make more sense as a master page to select the entire page then create the master page rather than clicking on an element and then creating a master page. Its confusing as I dont know if it will create a master page of the entire page and all the elements on it or just the element that is selected?