THe PDS slot extends the pins on the CPU out onto the motherboard so that CPU and certain other upgrades would have direct access to CPU functions, presumably to make them faster or give them greater controller over the rest of the machine.
On the beige PCI Powermacs, the slot that the CPU card plugs into, I guess would be the equivalent of the PDS on earlier machines, but I'm not sure if it really qualifies as it is the only means to plug a CPU into those models. All earlier models that had a PDS, had one CPU on the board and a PDS slot for a CPU upgrade.