Wow! I had no idea pipe makers were registering with the FDA. Makes me think the Agency may be closer to taking a harder look at pipes.
This whole concept of substantial equivalence comes from the medical device industry, which I actually have some experience with. If a new medical device is substantially equivalent to a device which already has FDA approval, the approval for the new device (called a 510k) is pretty streamlined. If your device has no substantial equivalent, you are subject to pre-market approval, a long and complicated process involving significant clinical trials. It looks like the FDA just copied this whole structure for tobacco products. As several of the posters have said, SE is an amorphous term and subject to very subjective interpretation. What struck me, looking at that FDA link, was that Sutliff filed a ton of applications for SE approval for things as petty as changing from a bag to a pouch or from changing from 1 ounce to 1.5 ounces (or vice versa, I forget which). But really petty stuff, as the tobacco itself was not changing. Easy things for the FDA to approve, but — thinking like a lawyer (always a dangerous thing) — if the FDA bureaucrats get it in their heads that SE is limited to such minor changes, perhaps they’d be more skeptical of changing from Cyprian to Turkish Latakia, for example, or changing the Virginia varietal to maintain a flavor profile. Or maybe I just worry too much. It’s a lawyer’s disease, which sticks with you even after you retire.