The External Bus inputs provide up to 64 MIDI input channels divided into four buses, each with 16 channels.
These MIDI inputs are primarily for controlling Reason Devices from an external sequencer.
This could be an external hardware sequencer or sequencer software that is installed on the same computer as Reason. You should preferably use a multiple port MIDI interface, so you can select separate ports for Reason and the other MIDI devices to use, although this isn't strictly required. See the chapter "Routing MIDI to Reason" in the electronic documentation.
The Remote Control input is used for assigning a MIDI port for receiving MIDI Controller messages. How to use Remote Control is described in the electronic documentation in the chapter "MIDI and Keyboard Remote Control".
Using MIDI Clock, you can slave (synchronize) Reason to hardware devices (tape recorders, drum machines, stand alone sequencers, workstations etc.) and other computer programs running on the same or another computer. MIDI Clock is a very fast "metronome" that can be transmitted in a MIDI Cable. As part of the MIDI Clock concept there are also instructions for Start, Stop and locating to sixteenth note positions.
By first selecting the appropriate MIDI input using the MIDI Clock pop-up and then selecting "MIDI Clock Sync" on the Options menu, Reason is made ready to receive MIDI Clock sync.
Reason normally tries to trim your computer system so that MIDI Input gets a higher priority than it normally does. This is to ensure best possible performance when for example recording notes via MIDI.
However, we can not guarantee that this attempt to boost MIDI priority will work on all systems and with all MIDI Interfaces. If you run into problems with your MIDI, try activating this switch.