Today a customer asked me how it comes that there is an iOS app for purchasing from Amazon while this seems to violate Apple's App Store review purchasing guidelines.

I could gave him no answer to his question.

It seems that there is a small number of allowed ways to do purchases in Apps that Apple permits, even if they do not get a revenue from.

So my question is:

What kind of purchases does Apple allow in an App that do not go through their purchasing system but through a custom one?