The PA Unit is a special case block, it will act like an interface, you can add processing patterns to it and as long as it is attached to the corresponding machine it will craft any pattern that is inside the machine. It's important to know this will only work for PROCESSING patterns.
Normal interfaces will do one craft at a time. If you have an or processing setup within your system the interfaces will pull out 1 ore and put them into the machine to make them into dust, it will then take 1 dust and put that into the next machine to be smelted.
The PA unit will take stacks of ore and dust rather than just one at a time. This is useful if you have a bunch of glass to be smelted down, waiting for 225 glas could take a while, using the PZ unit will speed this process up considerably. You can also use pipes on this block and have these pipes go to different machines, these pipes would need to have filters so there is no confusion what machine is getting what item to process.
You can also set inputs and outputs for each face of the block.