Great question.
When we say 1000 per user, that means that you can post 1000 DMs for each @handle that you are authorized to post on behalf of.
The 15000 per app means that – if you have several users that you are authorized to post on behalf of – you will only be able to post 15000 total for that app.
ie/ if you have 30 @handle authed for an app, you will be able to post 1000 DMs per user up to 15000 for that app.
Additionally, you should read up on our adaptive rate limits.
There will be a way to elevate these rate limits in the near future. We will make sure to update you when this becomes available.
There is no premium / enterprise access level for the DM APIs.