External event activities that needs input from an user through an UI when clicking on a task should be marked with this attribute and have Url to usercontrol specified. It is possible to specify a usercontrol for a task by setting property AssociatedActivity to a activity with this attribute. The given usercontrol should implement IWorkflowTaskControl

A workflow that want users to be able to set start parameters before it is used should be marked with this attribute and have Url set to a usercontrol. The given usercontrol should implement IWorkflowStartParameterHandler

Interface that usercontrols that are registered through use of attribute WorkflowPlugIn should implement. These controls are used to set StartParameters for a workflow definition that will be passed to instance when it is created.

Interface that usercontrols associated with a task should implement. The control can e.g. communicate with a custom event activity.