We have surrounded ourselves with three-letter acronyms—words made out of the initials of some three-word expression. RAM is random-access memory. CAD is computer-aided design. In American English, we usually set these acronyms in capitals to make it clear that they stand for longer expressions.
But not every little word with only three letters is an acronym. Sometimes it’s just a word. From a review of a digital camera:
Shutter Release LAG is the amount of time it takes to take the shot after autofocus.
“Lag” is not an acronym—it isn’t made up from the initials of anything. It’s just a word meaning “slowness” or “delay.”
English is full of these little three-letter words—cap, box, bit—and there’s a strong tendency, especially in technical text that’s already pockmarked with abbreviations, for writers to set them in capitals as if they were acronyms.
The cure for this tendency is to insist on knowing the meaning of every acronym you write. That will have the added advantage of making your writing clearer in other ways as well. If you don’t know what an acronym means, you really shouldn’t be writing about it as if you understood it, and you’re going to trip yourself—or your readers—eventually.