Solid State Logic SSL 18 User Manual page 22

At the top of the channel next to the meters, the 'DIR' (Direct To Bus) button allows each stereo Playback Return to bypass the main
section of SSL 18 Mixer's Routing Matrix and instead the signal is sent directly to the corresponding Aux/Bus Master.
In the above diagram, Playback 9-10 is highlighted in Blue to distinguish the difference between engaged & disengaged Direct
Buttons.
1. DIRECT TO BUS: MON L-R (Playback Returns 1-2)
Engaging the DIR button will send DAW Mon L/R outputs directly to the SSL 18 Monitor Bus (OUT 1-2), bypassing the Routing Matrix.
Enabling MIX ROUTES OFF will prevent any audio sent from the DAW or audio playback software on 'Outs 1-2' from reaching
physical Outputs 1-2. This is because playback stream 1-2 always passes through the SSL 18 Mixer at the final stage before reaching
the physical outputs.
2. DIRECT TO BUS: LINE 3-4 (Playback Returns 3-4)
Engaging the DIR button will send DAW 3-4 outputs directly to the SSL 18 Line 3-4 Aux Master (OUT 3-4), bypassing the Routing
Matrix.
Enabling MIX ROUTES OFF will prevent any audio sent from the DAW or audio playback software on 'Outs 3-4' from reaching
physical Outputs 3-4. This is because playback stream 3-4 always passes through the SSL 18 Mixer at the final stage before reaching