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".