Last Friday the FDA released its draft guidance on “Clinical and Patient Decision Support Software.” So why, you ask, is this document so significant? Because, as CNBC’s Chrissy Farr reports, the lack of certainty about what the FDA will regulate and what it won’t with regard to digital health applications has been keeping some developers—from giants like Apple and Google to a bevy of garage-based startups—in limbo. And it’s hard to do pretty much anything else when you’re limboing.

The FDA has actually been ruminating over this challenge at least since the turn of the millennium. That’s because the questions that need to be answered are far meatier than they might at first seem, as researchers Roberto Rodrigues, Petra Wilson, and Stephen Schanz explained in a comprehensive 2001 paper:

When does the amount of automated “clinical thinking” done by the software application constitute the “practice of medicine” with all its associated ethical and legal aspects?

To what extent does the application software allow clinicians to examine the underlying logic and to independently evaluate how the software arrived at particular conclusions?

How are the consequences of eventual errors to be dealt with?

Yes, big thinkers were asking those questions 16 years ago and have yet to resolve them. Now throw in a semantic one, which likewise has critical relevance to government regulators: When is the software or algorithm powering a medical device a medical device itself?

For wonky types—and I include myself in that category—the FDA’s litany of examples that constitute devices (and not) is fascinating.

Software that flags potential drug interactions and known drug allergy contraindications (based on FDA-approved drug labeling)? Not a device, according to the draft guidance document.

“Software that analyzes sound waves captured when users recite certain sentences to diagnose bronchitis or sinus infection”? Device.

For all the FDA’s Solomonic judgments, though, there are—and will be more—questions from product developers about inventions that fall between the cracks. One pressing question concerns the latest generation of fitness trackers, particularly as their technology is developed to enable sophisticated diagnoses.

In the draft guidelines, for instance, the FDA says “software that analyzes multiple physiological signals (e.g., sweat, heart rate, eye movement, breathing—from FDA-regulated devices) to monitor whether a person is having a heart attack or narcolepsy episode” is….a device subject to regulation.

So does that mean that a Fitbit (or Apple Watch) that can potentially warn of a dangerous heart arrhythmia—and maybe even help you avoid a stroke, as I wrote about here—is a medical device under the purview of the FDA?

Hmmm. If so, that might slow down the innovation train a bit. But then, I’m guessing that this latest FDA draft may get a few substantive revisions before all is said and done. That’s what drafts are for, after all. Watch this space for more.

This essay appears in today’s edition of the Fortune Brainstorm Health Daily. Get it delivered straight to your inbox.