initialisms

An initialism is similar to an acronym, but it is pronounced by its letters. 

Use a small s (no apostrophe) for plurals of most initialisms (e.g., PUCs and CFCs, not PUC's or CFC's).

Avoid the use of initialisms unless they are used extensively in a document. In short reports, spell out initialisms that are used fewer than five times. In long reports, spell out initialisms that are used fewer than 10 times. If initialisms are used, spell them out on first use, and put the initialism in parentheses after the full name.

To avoid confusion, try not to use too many in any sentence or paragraph. Include a glossary or list of acronyms if your publication contains a lot of them.

On the Web
The above guidelines apply to web content as well. However, if you use an initialism, spell it out or define it the first time you use it on each webpage. On the web, spell out initialisms that are used fewer than three times.