Most people already know that only the word "must" and not the word "shall" imposes a legal obligation on readers. If that's news to you, a past article covers that topic in detail.
What about the difference between "may" and "should?" The distinction between those two has far less serious consequences and is also easier to explain and accept. However, writing manuals and dictionaries don't say much about "may" and "should." As a result and without that guidance, I biased this report with many of my own thoughts and conclusions.
Nonetheless, it's reasonable to say that "may" means purely optional and does not imply that the writer recommends that option to the reader. "Should" also means optional but implies that the writer recommends and advises the reader to use that option.
Does it matter? Perhaps. If you write "should" to recommend that the reader use an option, then you probably have some protection if the reader chooses not to use that option and something goes wrong. But anytime you use the word "should," be sure you can say why you recommended that option and prove that your recommendation was valid and justifiable.
It's usually wise to ask your attorney for an opinion if you write official words that tell other people what to do. But generally if you include words of authority like "may" and "should" in your policies, directives or other instructions, then you're probably safe to use what's already in the FAA Plain Language Tool Kit (PDF) and Federal Plain Language Guidelines (PDF):
Must = mandatory
Must not = prohibited
May = optional
Should = recommended
These are the only valid word choices to convey those meanings.
You can schedule free, custom training for your staff to put your policies, technical reports, correspondence or other documents into plain language. To do that, or for questions or comments about this article, please contact:
Dr. Bruce V. Corsino
FAA Plain Language Program Manager