cover letter vs covering letter

cover letter and covering letter both are nouns.

Word	Noun	Adjective	Verb	Adverb
cover letter	Yes	No	No	No
covering letter	Yes	No	No	No

As nouns, cover letter and covering letter are synonyms defined as:

cover letter and covering letter: a letter sent along with other documents to provide additional information

cover letter (noun)	covering letter (noun)
a letter sent along with other documents to provide additional information	a letter sent along with other documents to provide additional information