I’m guessing this scenario is probably as good an illustration as any.
At the beginning of my Book there are 4 pages; Half Title, title, Copyright, TOC. This basic structure is repeated throughout the book. So what I want to do is have a section, call it something like MyBook or maybe the title of the book, i.e. “Diet Recipes” and have it span all four of these pages so each page gets a header and footer with the title and a page number. Something like “Diet recipes / copyright…….3” etc.
So it’s the multiple files bit that I’m having trouble getting my head around. Can it be done?
Sections are a document level attribute. You can't have multiple docs in one section.
You can, though, perhaps use ID's Chapter designation (also found in the Numbering and Section options) if you can find a way to use the Chapter Number variable (not sounding too likely, though). There can be no more than one chapter number assigned to a doc, but multiple docs can have the same number.
I suspect that some ordinary Running Header variables in your footer on the master pages could solve this, though you might find you have to add some non-printing "tags" on some pages to trigger them.