Acronyms

The first time you use an acronym or abbreviation that may not be obvious, spell out the word or phrase and follow it with the shortened form in parentheses. For example: A local area network (LAN). Use the acronym or abbreviation in all subsequent references. However, in long manuals or manuals that include sections that may be read separately, you may want to repeat the full name and abbreviation the first time a term is used in each new topic.
When spelling out an acronym, only capitalize the individual words if they're proper nouns. Use all capital letters for the acronym itself.
Certain acronyms and abbreviations are so common that you never have to spell them out:
- IBM
- RAM
- PC
- US
Never use acronyms or catch phrases as verbs.
If an acronym is generally read as alphabetic letters, choose the article according to the first letter:
- An API
- A URL
Don't use apostrophes to form plurals of proper nouns, acronyms, and numerals. For example, the plural of FAQ is FAQs, not FAQ's.
Don't use periods with most acronyms or abbreviations for technical terms, standard nontechnical abbreviations, or abbreviations for units of measure:
- ANSI (acronym)
- MB (technical abbreviation)
- PhD (nontechnical abbreviation)
- ft (unit of measure)
In documentation, use lowercase and periods in a.m. and p.m. Don’t use A.M., P.M., AM, PM, am, or pm. For UI text, use AM and PM. Use a single space between the time and AM or PM. For example, 10:30 AM in UI text, and 10:30 a.m. in documentation.

Salesforce.com Style Guide for Documentation and User Interface Text