The Pulse Relay node sends 1 to the output when the input Set is set to 1 and sends 0 to the output when the input Reset is set to 1. When both Set and Reset are 0, the previous state of the output is mantained. Whenever the input Trigger receives a rising edge signal, Set and Reset are ignored and the output is toggled.
The Behaviour property determines whether Set or Reset must be dominant when both of them are 1.

 Behavior of Pulse Relay

Behavior of node when Set input is dominant

The Pulse Relay it is used to transform a pulse input in a toggle switch!
Set and Reset inputs are optional, if they are not connected to any other nodes, the Trigger input is the only input which controls the output.