Hi @bpmnUser,
have a look at this process model: http://www.bpm-guide.de/wp-content/uploads/2010/07/Engine-Collab.png.
An example for your use case is the collaboration between the pools of the 1st Level Support Agent and the Trouble Ticket System: The single user tasks in the trouble ticket system edit 1st level ticket consists of three steps for the user in front of the screen: Classify ticket, Handle 1st level issue, Document 1st level result.
And everything is well documented in a single collaboration diagram.
Hope This helps, Ingo