This section of the documentation provides a technical overview of how reFuse works. Although you may skip this section and just follow the setup instructions, you may find the information here invaluable for troubleshooting.
Let's examine the following diagram, which illustrates Reason, reFuse, and Pro Tools:
Because ReWire can handle transport control and audio routing, it provides an all-in-one solution for connecting Reason and reFuse. DirectConnect, being an audio-only protocol, must be joined by two MIDI protocols, MIDI Beat Clock and MMC (Midi Machine Control), in order to provide transport control between reFuse and Pro Tools.
Note that MIDI data can also be captured into Pro Tools from the onscreen controllers (mouse-controlled knobs and sliders) in Reason. This is discussed further in the MIDI Data Buss section of Using reFuse.
You can also send MIDI tracks from Pro Tools to Reason using an IAC buss. This is the one connection shown in the diagram above that does not involve reFuse as an intermediary. You can read more about this technique in the Reason Operation Manual, chapter 3, "Routing MIDI to Reason."