There is a wide range of events you are able to associate applets with. Most of the events of interest are those associated with the GroupWise message contexts, such as ‘Mail’ and ‘Appointments’. For example, Formativ lets you associate an applet with the ‘On Send’ event of all message types, giving you the capability to write applets that perform some action prior to a message being sent.
Have a look at the Programmers Guide. It contains a topic entitled ‘Integrating Applets with the GroupWise Client’ that should answer most of your questions.
Hope this helps.