Saving a section is a feature designed to help you copy and save prebuilt sections of a layout to the library. Then you can use them to create new layouts without having to build sections for headers, footers, or templates from scratch.
Save a section
Then a popup appearing allows you to rename the section and save it to the library.
Save the section in the library
Add a new footer layout or edit another one, click add a new section, and you'll see the saved section on the section library. Click on Insert to add the section to the layout.
Insert a section to the layout
The instruction of saving a section above is not only applied to the footer, you can also do the same for the header and template's layout.