ReWire allows you to run Reason together with another application, such as a sequencer or another DAW. With ReWire 2, you can send MIDI from the host application directly to devices in Reason. See “Routing MIDI via ReWire 2”.The External Control Bus inputs (set up on the Preferences - Sync page and in the MIDI In device in the hardware interface) let you send MIDI directly to the individual devices in the rack. This is mainly used if you control Reason from an external sequencer, etc. See “About the External Control Bus inputs”.