For best practice, this should never be left null or evaluated differently based on the user (e.g. via LocalUser). If this input is null and the node is parented under a user, then the impulse will be fired for said user. If this input is null and not parented under a user, or if this input evaluates differently for different users, then the impulse will tend to be fired for the first user that sees the change in value. This is usually the one who caused the change in the first place. This could cause inconsistent behavior unless carefully managed. If the desired behavior is for an impulse to fire by every user that detects a change for them locally, then use the local version of this node, which is linked at the bottom of the page.
Usage
For use on ProtoFlux:FireOnChange and similar nodes.