Using a or an article before unfamiliar or new termed acronym [duplicate]

I have seen in a number of places, such as this answer, that the usage of a or an in front of an acronym is dependent on the common pronunciation of that acronym.

I am confused for the following reasons:

• It is very possible that a certain acronym will be differently pronounced in two cases (based on geography for example) due to cultural or context differences.

• If I am terming a new acronym, do I get to choose how to pronounciate it as well? For example, say I am writing an academic article and have defined a new term for which I choose a new acronym, how do I determine the correct article?

A good example of the latter case could be derived from the world of computers. Say I am an innovator who just invented the Serial AT attachment SATA computer bus, and I am terming the acronym SATA. I may pronounce my new term either as saa-taa or ess-ay-tee-ay. For the first case, I will have to write a SATA bus while for the second, it seems I will have to chose an SATA bus.

Is there a rule of thumb here to make things clearer?

This seems to be a question about prescriptive grammar versus descriptive grammar. As the creator you can decide how you want your word (or acronym, or initialism) to be pronounced, but don't be surprised if the public end up pronouncing it however they feel it should be pronounced. Look at the case of gif - the creator expected it to be pronounced with a soft g, but many people pronounce it with a hard g anyway. And whether a or an is used will of course depend on the pronunciation.

For initialisms, choose "A" or "AN" based on the pronunciation of the first letter

The United Nations is abbreviated by the initialism "UN", i.e. the two letters are spoken separately, U and N. The U is pronounced "yoo" in English, so the preceding article would be "a", even though U is itself a vowel.

The choice of article depends on the pronunciation of U, and not on the word that follows.

A UN organisation is being formed to report on ...

I am pleased that there will be a UN investigation ...

The UK security service "MI-5" starts with a consonant, M. Because the letter M is pronounced "em", it is preceded by "an".

Bob strenuously denies being an MI-5 operative.

We calculate an L2-loss for this neural network

For acronyms, choose "A" or "AN" based on the pronunciation of the whole word.

If the letters are pronounced as if they formed a whole word, rather than as independent letters, it is called an acronym.

For acronyms, the pronunciation of the word spoken is the key.

Here in the UK, UNESCO is pronounced "yoo-nes-co".

I saw a UNESCO report.

If in your region you would say "oo-nes-co", then it is perfectly reasonable to say (in speech):

I saw an UNESCO report.

However, when writing for an international audience, it is advisable to use the article that fits the typical US/UK usage, which would be "a UNESCO". This will look natural to most readers, and your local readers will not find it surprising since they will be used to seeing that particular formation and mentally converting it to "an UNESCO".

If the word could be an acronym or an initialism

For SATA, which could be pronounced S-A-T-A (initialism) or "saa ta" (acronym), you get to choose. In fact you get to choose regardless of whether you are the originator or not.

So by all means choose what seems more natural to you, but don't be upset if some later writers choose differently. They will choose what sounds more natural to them. We shouldn't get upset or possessive about these choices in writing as they are simply to help the spoken version be fluent. For example, a writer in a region where UNESCO is generally pronounced "oonesco" might choose to write "an UNESCO commission". It would look a little odd in isolation, if presented to a UK/US audience who pronounced it "yoonesco", but might be reasonable if it was known to be a transcript of a speech of someone from a region where it is pronounced "oonesco".