Here is how I understand it. Anyone feel free to correct me if I'm wrong:
Junior Manned - Assigned to fly a trip on a day that you were scheduled to be off and you are the most junior pilot available and eligible for the assignment. This classification normally applies to a line holders and not reserve pilots.
Junior Assigned - a position on an aircraft needs to be filled and no one bids for it. The most junior pilot on the seniority list that is available for reassignment is "forced" to accept the new aircraft assignment.
I say again that this is Junior Manning as I understand it at my company. If anyone knows differently, please feel free to let us know.
Junior Manning is a tool used by schedualing to make up for the lack of planning by crew planning. Junior manning was intended only for emergencies when the company lacked a crew member. It has become, at least at my airline, a crew planning tool.
For example, we still have 6 FO furloughed, but junior manning is still a common occurance.
Auto transfer you are mostly correct. Companies are only allowed to junior man if all reserves has been used also.