When building an acronym, there are occasionally small words that can "slip through the cracks". For example, U.S.A actually stands for United States of America. NASA stands for National Aeronautics and Space Administration. The word 'the' is another one that is left out here and there.
Is there a definitive list of words that are allowed to be left out of acronyms? Is it all just subjective and a case-to-case basis? I'm asking because I'm currently working on an acronym, and I don't think the word "with" requires it's own letter.