It is not actually a full bus (i.e. a given pin is not connected to 'all' instances of that pin, in every slot in the backplane); rather, slots are connected together in pairs, and these connections are used to connect together board pairs, without the need of a so-called 'over the back' connector cable.
In general (see below for details), the bottom (2, or solder-side) pins on one slot are connected to the top (1, or component-side) pins on the next slot. It runs down the right-hand side of the backplane, when facing the side of the backplane where the boards plug in.
Use by PMI
NOTE WELL: For reasons which seem utterly incomprehensible, many boards designed for Q/CD slots (such as PMI memory cards) do not avoid the QBUS pins on the CD connectors of a Q/Q backplane which contain 'hazardous' (to TTL circuitry) voltages. (The exact details of why are as yet unclear.) [NOTE: The exact failure mode here is still not understood; the PMI spec was examined, but no clash of pin assignments was found. The warning is accurate, though: MicroNote 28 says "MSV11-J MODULES CAN[NOT] BE PLACED IN A Q/Q BACKPLANE SLOT. IF THIS IS ATTEMPTED PERMANENT DAMAGE WILL BE DONE TO THE BOARDS".] So, plugging such a card into a Q/Q backplane will generally destroy the card.
The connection details are that for backplane slot 'N', in connector C, pins Cx2 are connected to the Cx1 pins in slot 'N+1', for x = B, D-S, and U-V. Similarly, in connector D, Dx2 is any slot is connected to Dx1 in the next slot, again for pins B, D-S, and U-V.
In addition, in slot 'N', in connector C, pin CA1 is connected to pin CC1 in slot 'N+1', and likewise in connector D, for DA1 to DC1. Finally, in slot 'N', pin CT2 is connected to pin DT2 in slot 'N+1'.
Finally, power (+5V) is available on pins CA2 and DA2), and ground on CC2, CT1, DC2 and DT1.
The DEC H9273-A backplane (used in the BA11-N mounting box) and H9276 backplane (used in the BA11-S mounting box) are both full Q/CD backplanes (i.e. every slot is a Q/CD slot); the former is Q18, and the latter Q22, although the former is easy to modify to Q22.
The DEC H9278-A backplane (used in the BA23-A mounting box) is a hybrid; the first three slots are Q/CD, and the rest are Q/Q.