This section lists the recognized exceptions to the In-App Purchase requirement — physical goods and services, person-to-person services, and enterprise software have always been allowed to use other payment methods; the exceptions are specific, not a general workaround.
Comes up when a developer assumes their app qualifies for an exception, such as treating digital credits as a "physical good," when Apple doesn't view it that way.
"Your app uses a payment method other than in-app purchase for content that does not qualify for an exception under guideline 3.1.3."