When to use Fixed Layout (FXL)

Using a Fixed Layout (FXL) format

* If your book is composed entirely or in great majority of images and/or if the text is strongly connected to the image (e.g. if the text is superimposed on the images or if the image is the background of the entire page of text), then you need a fixed layout format.

* If you'd like to publish:

a fairy tales book, with a specific page layout, with text superimposed on images, or with pages oriented horizontally;
a photo book or photo album;
a comic book
a book of mathematics or other sciences with a lot of algebra symbols, graphics, tables, etc.
a music sheets book, or similar
a book with specific symbols or alphabet less widespread, etc.
a book with the text translation facing an other language version...

For most of these cases you'll need a Fixed Layout book if the specific content (music sheets, graphics, symbols, etc.) of your book represents the majority of the content but if these elements are only a minor part of your book you'll probably be able to use a reflowable one, where you can still put images and special elements as long as they can "flow" on the layout of the page to adapt to the device of the readers.

Avoiding FXL:

A fixed Layout formats limits places where your book can be read and can lower the final output. That's why you should avoid using a Fixed Layout format if:

your book is only made of text;
your book contains images or tables but alternated with the text and not superimposed (with a linear connection, like: image, caption, text, etc.)
your book doesn't have a specific layout: no horizontal pages or specific disposition on a page that is important to keep as is for the global signification or feeling of the book;

To make things clearer, fixed layout are not recommended for most fiction and non-fiction books and apply only to very specific cases; to dig into the reflowable formats, take a look here.