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