This question comes up regularly on social media. I’m not sure if that’s because it is actually that confusing or because people simply don’t read closely.
If the term in question begins with a consonant sound (not necessarily a consonant! It’s about the sound, not the letter!), use “a,” like this:
A friend who applied to be a CIA operative used me as a reference.
We use “a” because we say “see eye ay,” which begins with an S sound.
Look again at “an S sound.” Remember, it’s about the sound of the beginning letter, not the letter itself. Because “ess” (what we call that letter) begins with a vowel sound, we use “an” with “S.”
With the initialism “NYC,” I see both articles used even in text produced within the city itself. The article guides me, as a reader, toward the expected pronunciation. If I see “a NYC bistro,” I know I’m meant to read that as “New York City.” If I see “an NYC bistro,” I’m meant to read that as “an En-Why-See bistro.” People who live in NYC have strong opinions about which is correct. (Which way did you read it that time? There’s nothing to guide you; there’s also no right or wrong answer. At least not to me, there isn’t.)
Recently this question arose about the term “FAQ.” Within the IT community, it’s universally accepted as a word, pronounced like “fack”: Read the FAQ. Outside that group, opinions and practices vary. Some people say “eff-ay-cue,” which would necessitate using “an” if one required an indefinite article. Others say “fack,” which of course would take “a.” I wager nearly everyone who has ever ordered anything online has encountered “FAQ” at some point, usually in wording like “Questions? Read our FAQ before contacting us.” It’s not so much unfamiliarity with the term that’s the issue. It’s whether you’re inside the IT community, where it’s just a word and pronounced like one, or outside, where you don’t hear or use it regularly. (Disclosure: My spouse has decades of experience in IT. I learned early on that it’s “a FAQ.”)
Remember: It’s the sound of the letter, not the letter that makes the sound, that matters in choosing the indefinite article that will guide readers to the intended pronunciation.