NFPA 1127 2002 edition. A copy of which is sitting right in front of me says:
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3.3.15.1 High Power Rocket. A rocket vehicle that (1) is powered by one or more high power rocket motors......
3.3.17.1 High Power Rocket Motor. A rocket motor that has more than 160Ns but no more than 40,960Ns of total impulse, or an average thrust greater than 80N, or more than 62.5 g of propellant, and that otherwise meets the other requirements set forth in NFPA 1125.
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NFPA 1122 says basically the same thing.
There is also this from the NAR web site:
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1. Who Needs High Power Certification?
A person needs high power certification if he:
1. Launches models containing multiple motors with a total installed impulse of 320.01 Newton-seconds or more, or
2. Launches models containing a single motor with a total installed impulse of 160.01 Newton-seconds or more, or
3. Launches rockets that weigh more than 53 ounces (1500 grams), or
4. Launches models powered by rocket motors not classified as model rocket motors per NFPA 1122, e.g.:
1. Average thrust in excess of 80.0 Newtons
2. Contains in excess of 2.2 ounces (62.5 grams) of propellant
3. Hybrid rocket motors
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https://nar.org/hpcert/NARhpdetails.html
I do not know how or why the 62.5 gram limit was added to the NFPA codes. But it always surprises people to learn that motors like the G33 and G75 are in fact high power rocket motors requiring user certification.