Litotes - Wikipedia, the free encyclopediaIn rhetoric, litotes is a figure of speech in which the speaker either strengthens or weakens the emphasis of a claim by denying its opposite. The literal meaning of a litotes is "not X (but not necessarily Y)", and a litotes can be used as an understatement, actually meaning "very much Y," or to express ambivalence.
So you can use litotes to express understatement, but not all litotes is understatement and not all understatement is litotes.
I am not ignorant of The Iliad. (litotes expressing an understatement)
I know a fair amount about The Iliad; I've been studying it for 20 years. (understatement but no litotes)
(There are lots of other examples in the Wikipedia article.)