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Having made (in vain) a good faith effort to answer this question without troubling the esteemed community, I pray the following expert-level advice on what I think all will agree is a thought-provoking matter:
What type of statement do you refer to if you call something by what it is not, like:
"That is not good" instead of "That is bad"
This is called negative strenghtening by Larry Horn in his classic 1989 book A natural history of negation. Horn notes that the phenomenon is conventionally classified as a case of litotes (emphasis through understatement).
Generally, it belongs to speech called circumlocution (talking around something); if the idea is to lessen the impact of the negative - say, telling someone their idea is bad - you could classify it as a euphemism or understatement.
Also relevant, if conditions are involved, is the idea of contrapositive. A contrapositive is an equally-true restatement of a condition in negative terms. E.g., "if something is a bat, it is a mammal" has the contrapositive "if something is not a mammal, then it must not be a bat."
