When you import planners or other documents into your PDF markup app (Goodnotes, Notability, etc.) the first page of the document ends up becoming the “thumbnail” or cover image in your document library within the app.
Replacing this first page with another decorative page will replace that cover. This is just a fun, decorative option that gives you the ability to add a little extra personality to your planners and notebooks.
Alternate covers are delivered as PDF files, just choose the decorative cover page you want to use, copy the page, and paste it as the first page in your planner or notebook document.