Learn English – n abbreviation for “skill”

The main reason for which this method was created is to avoid repetition of the full name.

You will write first the full name followed by the abbreviation in parentheses. Later on, whenever want to use the term in text, write only the abbreviation.

Recently, I have found an useful link to the American Society for Microbiology’s site where are discussed abbreviations and conventions:

Define each abbreviation and introduce it in parentheses the first time it is used; e.g., “cultures were grown in Eagle minimal essential medium (MEM).” Generally, eliminate abbreviations that are not used at least three times in the text (including tables and figure legends).

This is, at least in part, a question of style. Or the style guides of, say, news organizations.

• When you pronounce it as single letters, such as CNN (see en en) or BBC (bee bee see) it is usually written all caps.
• When you pronounce it as one word some organizations write only capital letters, some spell it like a proper noun, first letter capitalized, rest lower case: CNN writes NATO, BBC writes Nato, as you say nato, not en ay tee oh.
• There are lots of exceptions. SCSI is pronounced like a word, SKUZ-ee, but spelled upper case by the BBC, too.