Various particle operators can create "events" when something happens, such as a collision, or a value passing some threshold. An event can trigger the activation of other particle operators, allowing for complex and interesting behaviors.
Downstream you can use the popevent function to detect a named event. This is very useful in the Activation parameter of a POP node, where you can trigger the node only if the event has fired.
Events are identified by a name string.
The info button on the POP node’s tile gives information on which events are currently occurring.