Thanks for letting me know about the dead link. Will check with the author if it's available somewhere.
"there will always be a rom file and a CHD, but never just a CHD for one particular game."
erm.....not exactly.....again....a chd is simply a compressed container for a binary image (which can be anything from a floppy to a laserdisk etc). It depends on the system. As ROM file you usually take a dump of an IC based on the boards of the machine. ICs holding gfx, sound data and program code, etc.
There are some set definitions in MAME which only consist of a CHD...but in reality the underlying system is represented by a bios set, too, which is a rom dump.
So...anyway, it actually does not actually matter. Best thing is to go with this:
A set is a collection of files which represent the (usually) game. A set includes none, one or more of rom files, sample files, chd files (but at least one of them).