The SBC6120 is a single board computer using the Harris HD-6120 CMOS “PDP-8 on a chip”. See Intersil 6100 on Wikipedia.
The SBC6120-RBC Edition is a re-implementation of the SBC6120 design into KiCAD as a 2-layer PCB layout. The original 4-layer PCB is relatively expensive to fabricate, which makes it difficult to distribute as a “hobbyist PCB” in the way the other RBC ECB, S-100, etc boards are distributed. Having the schematic and PCB layout in KiCAD will also benefit other community members who might want to make use of portions of the design which Bob has graciously made available.