How do you know what FLIS and FCIS are about?
Well, the FCIS I know because it contains a field with the content length as well as a field that indicates the number of images available in the file. It also grows and shrinks depending on how many image files are present in the file so it suggests it is something of an information record where most information is for the images.
I really need more mobipocket files before I can be sure about the FLIS because I actually only have a batch of DRM'less versions I test with. However it is a static sized record with remarkably much 0x0's in it and 0xFFFFFFFF's suggesting unused information in the record. And since all the other relevant information is elsewhere it seems very probable it is for the DRM. As mentioned, I ened a DRM'ed file to be sure.