To configure Receive Notification, Wait, and Notify activities, you must specify a key that correlates Notify activities with Receive Notification or Wait activities. The key is a string that corresponding activities specify to determine when a Receive Notification or Wait activity should accept data from a Notify activity. The key is similar to event keys used in activities that wait for incoming events (described in Event). The key is a string, but you can use any XPath expression that evaluates to a string when the process instance executes.