on The Book Designer:

In my discussions on ebook creation, I’ve largely focused on what are called reflowable ebooks — books designed like web pages to be responsive, adjusting to the size of the screen they’re being displayed on, the preferences of the reader, etc. Page breaks and image sizes may change radically, depending on whether the ebook is being read on a phone, a tablet, or a full-sized monitor. For most books, this is a great way to present the text — in the manner that best suits your reader. The vagaries of the reflowable ebook, however, do introduce uncertainty that doesn’t work for all ebooks.

But there’s a different kind of ebook where those kinds of uncertainties can be eliminated: the fixed-layout ebook.