FS2004 creates a temporary cache index file for every scenery folder - it is named scenery.dat. It references the .bgl files in the scenery folder and has some data about when they are used.
The problem is that FS2004 has a very bad process to clear old data from those temporary files. When people have problems like a scenery not loading properly, AI aircraft appearing thousands of miles off course, texture issues with some addons, AFCAD files acting funny - deleting the scenery.dat files will often resolve those issues.
One thing which does cause a problem is when people load a scenery file into a scenery folder and over write their scenery.dat with one which came from the developer's computer.
That might be what the folks are talking about.
Personally I delete all the scenery.dat files at least weekly and let FS rebuild them. Whenever I modify the files in my Addon Scenery folder, when I add new AI files to the Scenery\World\Scenery folder - I delete the scenery.dat files for those folders so that the new files will be properly recognized.
I never, ever, allow a scenery.dat file from any download into my FS folders.
@ PawPaw's house - near KADS, Addison, Texas, USA