What's special about the Pilz safety relays? Looking at their web site, it looks as if, electrically, it's "just" a relay. I'm guessing that it is specially built for reliability (no chance of welding contacts, that kind of thing?), and it's used as an alternative to wiring things like BOB enable out to the drivers and VFD, etc., to minimise the amount of circuitry involved in e-stop operation and hence improve reliability. In particular, it takes the control PC out of the loop.
Is this right? What have I missed? How important is it to fit something like this rather than rely on e-stop->BOB->PC->BOB->drivers?