Rules provides a number of events – places in a Drupal page load that can be used to fire off actions. Rules that trigger on events are called reaction rules (in contrast to stand-alone rule components that must be triggered by other means – see the section about components).
Each event may provide some associated variables, which can be used in the rule configuration. For example, the event After saving new content provides the saved content as a node variable while After updating existing content provides both the updated and original content as variables – thereby allowing comparison between the two.
A reaction rule may have more than one triggering event. If they provide different variables, only the variables provided by all events may be used in the configuration, assuring that the rule may be evaluated properly regardless of which of the events is acting as trigger.