The rules are complex but the principle is simple.

Apps are required to use in-app purchase if:

(1) The customer relationship originated in the app; and

(2) The primary nexus of the purchased item is to improve the purchaser's personal in-app experience.

Rules get complex around edge cases even when the principles are simple. Sometimes Apple makes the wrong call on those edge cases based on those principles (Hey, WordPress, in-app fitness training calls), and today's rule changes feel like Apple's acknowledgement of that.

Apple has also signaled that they don't feel the simple principles need any adjustment.