An acronym implies some form of abbreviation that itself becomes a new word.
Pedestrian light controlled crossing: pelican crossing (o shifts to a to satisfy the orthography)
Defcon comes from defense condition in the US. There's no real reason why it should be written in caps.
An abbreviation is not always an acronym. DWP: department for work and pensions.
There are contracted forms however that will often be said other than as they appear:
etc. - said as et cetera
N.B. - often said as nota bene
text abbreviations too will generally be "heard out full": IMHO - in my honest opinion.
So, although we write in order to suggest a voice in the reader's head, how we write something isn't always how we "hear" it.
The a/an article shift occurs in accordance with how the writer expects the reader to "hear" what is written. It's not predicated on vowels and consonants as such, but on vowel sounds, and consonant sounds. So letters that start with an "e" will take "an": F, M, N, S, X.
An X-rated movie.
An MP in a UK assembly.
If "SAP" is known both as its letters and as an acronym (word), the writer will indicate their choice in the article:
An SAP procedure is an S-A-P procedure.
A SAP procedure is a SAP( or "sap") procedure.
To write the acronym NATO and expect the reader to hear "North Atlantic Treaty Organization" is going too far. The writer can write either, depending which they want the reader to "hear" in their own mind. But they can't write the one and expect the reader to hear the other in their mind.
