There are many ways to monetize your app on the App Store. If your business model isn't obvious, make sure to explain in its metadata and App Review notes. If we can't understand how your app works or your in-app purchases aren't immediately clear, it will delay your review and may trigger a rejection.
3.1 Payments
3.1.1 In-App Purchase
3.1.1(a) Link to Other Purchase Methods
3.1.2 Subscriptions
3.1.2(a) Permissible subscription uses
3.1.2(b) Subscription Upgrades and Downgrades
3.1.2(c) Subscription Information
3.1.3 Other Purchase Methods
3.1.3(a) Reader Apps
3.1.3(b) Multiplatform Services
3.1.3(c) Enterprise Services
3.1.3(d) Person-to-Person Services
3.1.3(e) Goods and Services Outside of the App
3.1.3(f) Free Stand-alone Apps
3.1.3(g) Advertising Management Apps
3.1.4 Hardware-Specific Content
3.1.5 Cryptocurrencies
3.2 Other Business Model Issues
3.2.1 Acceptable Business Models
3.2.2 Unacceptable Business Models