To catch s.t.; to receive s.t. (like a letter, gift, money, dowry); to accept s.t. that s.o. hands to you.
When the -an imperative is used the focus is on catching s.t. so it isn't lost or wasted. Thus you are in a sense affecting the object by changing its wasted state. However you don't catch all of it, only some. When -un is used you focus on catching or receiving all of s.t. and not for the purpose of saving s.t. from being wasted.
That person is really skilled at catching a ball.
Catch that rain water in a basin.