songbird29,
is there a hierarchy of data-sources
Yes.
An SSR return is a circle. Primary return is +. If you have both, then the + is inside the corresponding SSR return. Other than this you will only see one symbol. Hence there is a hierarchy. SSR/PSR, ADS-B, ADS, Flight Plan track.
But why then would you need 50NM separation when the data source for the position is ADS
You need the 50nm separation because of the delay in receiving the information from the aircraft. It can take up to a few minutes to get the request (one shot) to the aircraft and for the aircraft to send a response. The controller could be sitting in Melbourne or Brisbane, and the aircraft up to 10hrs flight time away across the other side of the airspace. Remember Oz controlls a lot of oceanic airspace where the ADS comms are by satelite.
The controllers air situation display then provides a computer extrapolated position for where the aircraft should be based on the aircraft's response.The ASD provides an updated position every few minutes (which is also computer extrapolated) without the aircraft actually sending it's position information.
The aircraft automatically sends it's position info over every position report, and also at 30 minute intervals (ADS contracts for position reporting depend on where the aircraft is eg. off track, oceanic, under radar, etc). So the position of the aircraft displayed to the controller is accurate to a degree The separation standard has added significant buffers for safety. Remember, outside VHF coverage these aircraft are also communicating by HF or CPDLC which takes time to process by controller and/or pilot.
ADS-B is intended for use with mainly aircraft in VHF coverage.
Hope this helps.