modal verbs - SHALL vs. MUST in technical documents

In my company, we often need to write requirements for our customers. They are pretty much recommendations (i.e., we do not impose anything), but given a certain baseline, some are stricter than others: they can be mandatory, desirable or optional. To avoid misinterpretation, we use the terms defined by RFC 2119. This question focuses on the first definition:

1. MUST This word, or the terms "REQUIRED" or "SHALL", mean that the definition is an absolute requirement of the specification.

So, in principle, either of the three words could be used exchangeably. I tend to use must, whereas some colleagues tend to favor shall. For example, I'd write

Every car MUST have a key.

and others

Every car SHALL have a key.

However, the latter (shall) resonates odd, unnatural and even pretentious to my ears. From other questions in the site and my previous experience, I get different ideas:

• It sounds refined, but old fashioned.


• As a question, it is just a proposition (shall we go later to...?).


• It is seldom used, but in laws and other official documents.

So my question is: is there something that speaks in favor or against using one or another option (i.e. MUST or SHALL)? And slightly related: am I being too critical of this usage of SHALL in modern English?

Answer

The simplest way of indicating a requirement, as per the RFC, is the use of the word "must". The alternatives mentioned in the RFC exist to allow for slightly more natural English avoiding the repetition of the word "must"

"Every car MUST have a key, which shall be made of metal and is required to weigh less than 100g"

The RFC was trying to codify existing practice, in which words like "shall" were already in use. The trouble with shall is that it can indicate the simple future (usually with a first person pronoun). Saying "The car shall be delivered on 10 July" could be an obligation or it could be a rather badly phrased prediction (will would be better here). Technical specifications don't need to be great prose, so just use "must"

There are a couple of notes about your question: You say "They are pretty much recommendations (i.e., we do not impose anything)". The purpose of the terms "MUST" or SHALL in the technical language is to impose obligations. The car MUST have a key, or else you don't get paid. For recommendations you use SHOULD "The car should be painted blue" (unless there are good reasons not to.)

Also, you say that you are writing these "for customers". Normally it is the customers that write the specification. Do you mean "for suppliers" or are you actually talking about technical documentation, for which the specification language of RFC 2119 is not appropriate.