A bulk packaging has to show which material is inside by its identification number. On which faces does that number go, and what decides it?

Explanation

On a bulk packaging the identification number goes on each side and each end when the packaging has a capacity of 3,785 L (1,000 gallons) or more, and on two opposing sides when the capacity is less than that. Capacity is the whole trigger here, which is what separates this rule from the placarding rules: a placard requirement turns on the material and often on aggregate gross weight, while this marking requirement turns on how much the packaging can hold. Capacity decides it in the two cases above, but the paragraph lists a third alongside them and sets no capacity test for it: for cylinders permanently installed on a tube trailer motor vehicle, the number goes on each side and each end of the motor vehicle. The identification number is the figure responders match against their own references, so the rule puts it where it can be read from more than one approach. The wrong options each borrow the wrong trigger or the wrong faces, load state rather than capacity, a front-and-rear placement the subpart never uses, or a flat two-sides rule that would make capacity irrelevant.

Source: 49 CFR 172.302(a)