used to state that you are not suggesting sth 倒不是；并不是说
She hasn’t written—not that she said she would.
她还没写信来——倒不是她说过她要写。
I tried to help, not that it mattered. [=but it did not matter]
我尽量帮忙，但这不是什么大事。
I saw him with some other woman, not that I care. [=but I don’t care]
我看见他和别的女人在一起，但我并不介意。