When One Must Know Should from Shall or May

When reading, and correctly interpreting, the intent of the Hazardous Materials Regulations (49 CFR) one must be careful to understand the “Rules of Construction” which refer to the context.
Per 171.9(a), “Words imparting the singular include the plural. Words imparting the plural include the singular; and words imparting the masculine gender include the feminine.” Additionally, per 171.9(b), “Shall” and “Must” are used in an imperative sense whereas “Should” is used in a recommendatory sense and “May” is used in a permissive sense. “Includes” is used as a word of inclusion, not limitation. The applicable criteria for the HMR does include all of these words that, when used in context, must be meticulously understood. References that use “Shall” and “Must” are to be interpreted as requirements. References using “Should” are not required, but greatly preferred. And a reference using “May” gives the shipper the option to use their own judgment regarding compliance. You may want to revisit this paragraph because you must know what should or shall be required when a shipment includes a regulated material.