letter vs textual matter

letter and textual matter both are nouns.

Word	Noun	Adjective	Verb	Adverb
letter	Yes	No	Yes	No
textual matter	Yes	No	No	No

As nouns, textual matter is a hypernym of letter; that is, textual matter is a word with a broader meaning than letter:

Other hypernyms of letter include document, text.

letter (noun)	textual matter (noun)
a written message addressed to a person or organization	the words of something written
the conventional characters of the alphabet used to represent speech
an award earned by participation in a school sport
a strictly literal interpretation (as distinct from the intention)
owner who lets another person use something (housing usually) for hire

letter (verb)	textual matter (verb)
mark letters on or mark with letters
set down or print with letters
win an athletic letter