Magento has two relevant payment methods: Authorize.net and Authorize.net Direct Post. My assumption is that the Authorize.net method uses the AIM api, and the Authorize.net Direct Post uses the DPM api.
A comparison of the Authorize.net APIs can be found here: http://developer.authorize.net/api/compare/
The code seems to support my assumption based on these two models:
Can anyone confirm/deny this assumption?