DOCBOOK: imagedata: entityref vs fileref

Greetings,

I'm new to the list and I have a question regarding the use of the
entityref and fileref attributes when including an image via the
imagedata element. When do I use which attribute? Is it merely personal
preference? What are the benefits? What are the drawbacks? What is the
preferred way?

The processing expectations say:
> There are two ways to provide content for ImageData: EntityRef or FileRef. It is best to use only one of these methods, however, if multiple sources are provided, EntityRef will be used in favor of FileRef.

How do I have to understand 'if multiple sources are provided' - if I
specify a fileref and an entityref attribute for the same imagedata
element?


Thanks for your enlightenment,
Patrick