A full list can be found here (on their helpful, seemingly recent, marketing site): https://www.apple.com/ios/app-store/principles-practices/#:~....
And the legalese is here in section 3.1: https://developer.apple.com/app-store/review/guidelines/#in-...
For a few categories, this is very well-defined. For example, a ride in Uber/Lyft is clearly a good/service consumed outside of the app, a subscription to Netflix is a subscription, and a +1 life in a game is a consumable digital good. However, for other categories it is not as clear; for example, what category does a "Ride Pass" fall under (which lowers the cost of Uber and Lyft rides, but is a monthly, purely-digital subscription)?
Ads as a whole probably gets bucketed into goods and services outside the app (because it doesn't improve your app experience). Beyond that, from a strategic perspective, I suspect Apple isn't trying to get into the b2b game, their ability to control supply there is probably much lower, esp when the developer and the user have a direct connection (through a sales person / customer success rep). However, this raises a really interesting question towards the category of lighter-weight personal ads (i.e. "Promote this post"). I suspect that these are not well regulated and probably not equally enforced. I'd be interested in the comps here: Do Twitter / FB / LinkedIn all use the same purchase method when you promote a post? How about the dating apps when you promote your profile? Are those materially different?