Sometimes it’s left to me to address the big questions.
Consider the defined terms SoW (used to meaning a statement of work issued under a particular contract) and MAC (used to mean a material adverse change in something or other). (MSCD 9.73 recommends that you use Material Adverse Change as the defined term, not MAC, but MAC provides a convenient example for purposes of this analysis.)
The question is, do you say a SoW or an SoW? And do you say a MAC or an MAC?
That depends on two factors. First, is the defined term an acronym or an initialism? And second, does the defined term begin with a vowel sound when you say it?
Regarding the first point, here’s what Garner’s Modern American Usage 2 (3d ed. 2009) says:
An acronym is made from the first letters or parts of a compound term. It’s read or spoken as a single word, not letter by letter (e.g., awol = absent without official leave, radar = radio detection and ranging, and scuba = self-contained underwater breathing apparatus). An initialism is also made from the first letters or parts of a compound term, but it’s sounded letter by letter, not as one word (e.g., r.p.m. = revolutions per minute).
If treated as an acronym, it’s likely that SoW would be sounded out so that it rhymes with now (as opposed to rhyming with doe). But it seems particularly unpromising to use as an important contract term a homophone for the word for a female pig. So I suggest that SoW is best treated as an initialism. That would result in it beginning with the same vowel sound as egg. It follows that you’d say an SoW, not a SoW.
What about MAC? Invariably, deal lawyers treat it as an acronym—I’ve never heard anyone say em-aye-see. So you should say a MAC, as MAC begins with a consonant sound if you treat it as an acronym when you say it. You can find instances of an MAC on the SEC’s EDGAR system, but they look odd.
That is all.