If you sell content, e-books, coins, workouts or anything similar you must use the in-app purchase system.
Here are a few examples:
- Selling E-Books, PDFs, Videos, Audio, Pictures or similar
- User needs to pay to access special content / parts of your app
- You are selling coins or points that can be used in your app for other digital goods
- You are selling subscriptions to get more "features" or content in your app
Here you can find more information about the guidelines of in-app purchases: