For my clarification, just because I'm suddenly interested:
* US Law requires "certification" to buy high powered rocket motors (Above G class)
* Only the National Association of Rocketry and Tripoli Rocketry Accusation currently issue those certifications in the US (info from here: https://www.nar.org/find-a-local-club/section-guidebook/laws-regulations/)
As long as you aren't using "High Power Motors" (any motor whose average thrust is above 80 Newtons, or above G class) you don't need their certs.
So the "no guidance" thing is more of a "keep the club happy to keep your cert" guideline rather than a hard "law"?
US law does not require operator certification. High power rocketry is self-regulated by the two major national rocketry organizations, the National Association of Rocketry, and the Tripoli Rocketry Association. They handle all high power certification in the USA. Class 2 and Class 3 rockets flyers should be certified by one of the national associations, but their is no federal regulation or law that requires such.
FAA regulations do define what is considered a high power rocket and what requires a FAA waiver for launch. The waiver is to ensure the area is safe for other users of the airspace.
The waivers also require flights to be sub-orbital, or ballistic in nature. Different waivers are required for orbital flights, and no amateur has applied for one to my knowledge.
A Class 1 model rocket is defined under 14CFR 101.22 (a):
"Class 1—Model Rocket means an amateur rocket that:
...(1) Uses no more than 125 grams (4.4 ounces) of propellant;
...(2) Uses a slow-burning propellant;
...(3) Is made of paper, wood, or breakable plastic;
...(4) Contains no substantial metal parts; and
...(5) Weighs no more than 1,500 grams (53 ounces), including the propellant."
A Class 2 high power rocket is defined under 14CFR 101.22 (b) "Class 2—High-Power Rocket means an amateur rocket other than a model rocket that is propelled by a motor or motors having a combined total impulse of 40,960 Newton-seconds (9,208 pound-seconds) or less."
A Class 3 rocket is defined under 14CFR 101.22 (c) "Class 3—Advanced High-Power Rocket means an amateur rocket other than a model rocket or high-power rocket."