Explanation of structure and function of a safety relay
The Pilz unit, being referred to, is known as an 'E-stop safety relay'.
At the outset, the E-stop relay circuit comprised a relay controlled by an 'E-stop' push button switch and a 'Start' push button switch. It was generally referred to as the 'Control On' circuit.

This circuit could fail, should the 'NC' contact of the E-stop push button switch get stuck. Hence it was supplemented by a second 'NC' contact in series. It could also fail, should the 'NO' contact of the E-stop relay get stuck. So a second relay was added, with it's 'NO' contact wired in series with the first one. The resulting circuit was fail-safe but the fault could remain unnoticed. Hence the third relay was added to monitor the other two and prevent their subsequent turn-on.
The 'NO' contacts of the first two relays, wired in series with the 'NC' contact of the third one, ensured E-stop of the machine and prevented subsequent restart in case of a fault (with the status of the third relay also being monitored).
This dual-redundant circuit is the basis of the Pilz E-stop safety relay.
It is also used to render fail-safe functioning of machine guards, light curtains, motor clutch / brake units etc.