void setPresenceStates(Collection<String> presenceStates) {
        this.presenceStates = presenceStates;
    }
Returns the keyword that the event needs to match. When {@code null} all event
are going to be notified to the subscriber.

@return the keyword that the event needs to match. When {@code null} all event
        are going to be notified to the subscriber.
xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/NodeSubscription.java