This is an atomic concept (don't expect links out of here).

"If I have money, I'll go to the movies." And if I don't, I can still go to the movies by using a credit card. That could create some problems at the end of the month, oh well.

The point is: we say IfAndOnlyIf (sometimes abbreviated Iff) when one statement is equivalent to the other.

 IfAndOnlyIf A Then B

is equivalent to

 IfAndOnlyIf B Then A