As a note, the FDA circulars in particular (like the labeling one) aren't technically law, but because they're (1) put out by the FDA and (2) a good summary people treat them as law. The thought is that you can always point to the circular if you need to.
You should read the actual regulations as well, but that's work so lots of people don't do it.
Here's an example of why you should read the regulations. Technically, supplements can't make structure/function claims. However, you can submit your structure/function claim to the FDA. Once you do that you can make that particular structure/function claim on your labels. Since the FDA has no process in place to actually review those claims your structure/function claim is suddenly valid.
You should read the actual regulations as well, but that's work so lots of people don't do it.
Here's an example of why you should read the regulations. Technically, supplements can't make structure/function claims. However, you can submit your structure/function claim to the FDA. Once you do that you can make that particular structure/function claim on your labels. Since the FDA has no process in place to actually review those claims your structure/function claim is suddenly valid.
It's amusing.