LFD files contain various resources, mostly sound and graphics.
You don't need them to create a new level, but for things like cutscenes and briefings.
The Dark Forces LFD format seems to be completely compatible with X-Wing and Tie Fighter LFD files.

LFD_Ix_Entry IS { TYPE char[4] // type of the resource NAME char[8] // name of the resource LENGTH long // length of the resource }

Then LENGTH bytes follow the header.

The first index entry is of type RMAP, and contains the list of all the sections in the .LFD file.
This is similar to the GOB Master Index.

The other sections can be:

Section	Description
ANIM	animation, this is a collection of DELT
DELT	static image in delta format
FILM	'script' referencing the other resources in the lfd
FONT	font
GMID	General Midi music
PLTT	palette used for ANIM and DELT
VOIC	VOC (standard Creative Labs format)