A master page is like a document template — any object placed on a master page appears automatically on pages that are based on that master page. This is useful for creating consistent elements such as page numbers, logos, or placeholder frames. Each document can have multiple master pages.
In the Page menu of the Properties panel you will see a single master page and the pages in the document.
In the Pages panel (Window > Pages), the letter A on each page thumbnail indicates that the default master page, named “A-Master,” is applied to that page.
To add content to a master page, with no content in the document selected, choose the master page from the Page menu in the Properties panel.
A master item cannot be selected on a document page unless the master item is overridden.