The RC2014 system has not many specifications - its mainly the bus is everything depends on.
The "Standard" Bus depends on 1x40 pins slot.
The Enhanced Bus introduces an additional row at each slot - part not over the whole backplane - so its more a 1x40,1x24 slot definition. The following picture shows the Extended- and the Standard-Bus on the 12-slot BackplanePro
The 3rd generation backplanes will have 2x40 pins slots. They are compatible with the Extended Bus, but offer additional lines for signals. Up to now no public available module depends on this bus. The need for this bus is the wish for 16bit computing power or having more address lines available and for a correct IEI/IEO handling.
With the IEI/IEO problems it also showed up, that additional backplanes are needed to offer look-up support for more than 4 modules with Zilog devices. So we might see another generation of backplanes ...
Actually my boards are using some of the additional lines - for Interrupt-Lines (2 additional lines for Z180).