Long before there was a 921 in which you could write definitions and then cite them as proof, there was the science of explosives. The definition in 3.3.89 is appropriate for condensed explosives but leaves a lot to be desired with respect to gas mixtures.

It is, by the way, relatively easy to bring gasoline/air mixtures to detonation, but natural gas will not cooperate.

3.3.103 is also poorly applicable to gases.

I like 21.3. You should read it.

21.3.1 and 2 were obviously influenced by what might be described as "old wives tales" common among blasters. The prose muddles the the distinction between the fundamental dynamics of the explosive and the quantity thereof. Both of these sections are misleading if applied to gaseous explosives. The differences in the level of destruction described therein may be nothing more than a reflection of differences in quantity or degree of pre-mixing of the gaseous components

I suggest that in some future edition of 921 the "high order," "low order" verbiage be deep-sixed. The use of these terms by some members of the explosives industry has never been anything but ambiguous and confusing.