The SSL 18 Mixer runs off the on-board DSP to provide low-latency headphone mixes and many other versatile features such as mirror routes.
The DSP has a finite capacity and as such, a total of 4 pairs of Playback Returns from the DAW can be brought into the SSL 18 Mixer.
We decided that these pairs should be: Playback 1-2, 3-4, 7-8 and 9-10.
So why is Playback Return 5-6 missing? Why didn't we make it Playback 1-2, 3-4, 5-6, 7-8...
The reason is centred around making sure every customer can get the most out of SSL 18 and in particular, making sure Pro Tools users are not constrained by the way in which its hardware inserts work. In most DAWs, a hardware insert can be created using differently numbered outputs and inputs. E.g. You can send out of the DAW on 9-10 and then route back into the DAW on inputs 7-8 and configure this as a hardware insert path. With Pro Tools however, the output and input numbers must always match. E.g. to successfully create a hardware insert you would need to route out of 9-10 and route back into inputs 9-10 for Pro Tools to consider it a hardware insert.
Now, let's imagine a typical use case example for SSL 18 - outputs 1-2 feed the main monitors, outputs 3-4 feed alternative monitors, you also desire to have a pair outputs (and inputs) feeding analogue hardware and you also want to use the 2 independent headphone mixes for tracking. With SSL 18, the last 2 Playback Return pairs SSL 18 are used to create custom headphone mixes for HP A and HP B.
If, we would have fed these from Playback 5-6 (HP A) and 7-8 (HP B), then Pro Tools users would not have been able to use Outputs 9-10 to create a hardware insert path because Pro Tools doesn't allow the hardware insert input numbering to be different to the output numbering - so using outputs 9-10 with inputs 1-2 wouldn't be permitted by Pro Tools.
Therefore, with a total of 4 stereo Playback Return pairs being able to be routed into the SSL 18 Mixer, leaving out Playback 5-6, although initially seeming a little odd, was the best way of accommodating all DAW users for the most flexible range of setup options.