I found this nugget in the AUM FAQ:
"What about MIDI Clock?
Since version 1.2.1, AUM allows sending MIDI Clock to a single CoreMIDI destination. The purpose is to synchronize external hardware. The clock is aligned with the audio output of the device.
NOTE: Trying to use it for syncing other apps or via wireless MIDI will probably not align well! There’s no reason to continue using MIDI clock for sync between apps, when we have Inter-App Audio host sync and Ableton Link.
Inter-App Audio apps and Audio Units should implement host sync, which has been part of the IAA standard from the beginning, and is the natural way for a node to sync to the host (AUM in this case)."
For the price, I wish I could test AUM before purchasing. I bought Elastic Drums a week ago because I like to do synth drums instead of sampled drums, but now it just sits there because I prefer an Audio Unit with a MIDI piano roll, rather than flipping back and fourth between a DAW and a sequencer.