what does the term usage guidelines mean

Primarily, this term means the correct ways in which grammar and usage should be used when writing and speaking in English.

English Usage Guidelines and Recommendations
English Usage Guidelines and Recommendations. KDE documentation is written in standard US English (rather than any other regional variety of English). ...
http://l10n.kde.org/docs/doc-primer/guidelines-english-usage.html

The most famous book to help you with this is still Strunk & White's Elements of Style.

Strunk, William, Jr. 1918. The Elements of Style - 2 visits - Jul 9
For use in English courses in which the practice of composition is combined with the study of literature. It aims to give in brief space the principal ...
www.bartleby.com/141/ - Cached - Similar -
Rules of Usage. Strunk, William, Jr. 1918. Elements of Style
William Strunk, Jr. (1869–1946). The Elements of Style. 1918. II. ELEMENTARY RULES OF USAGE ... red, white, and blue. honest, energetic, but headstrong ...
http://www.bartleby.com/141/strunk.html

Amazon.com: The Elements of Style, Fourth Edition (0076092005445 ...
Strunk & White's "The Elements of Style" is a wonderful book, and it is great for learning grammar, in that it is witty and demonstrates the rules of ...
http://www.amazon.com/Elements-Style...Strunk/dp/020530902X