ROE Variations
Even with a command from Higher HQ to engage a certain aircraft it can be changed at a moments notice.
I have had experience of an initial "engage" for BVR to "engage" if identified as Hostile, to finally "engage" if hostile and seen committing a hostile act. All this in a peace enforcing role that had very restrictive ROE.
To continue 3.4 vein sometimes the ROE can be based on electronic means. For example in Veitnam the USAF had a piece of kit "Combat Tree" which could interogate Soviet IFF, therefore they were allowed to BVR if they had a psoitive reply from the Soviet IFF.
Most electronic ROE's are matched up with visual ROE's. But some can employ secret stuff that is supposed to guarantee a definate hostile ID. However if the oposition isn't radiating then it can't be declared Hostile and Visual ID will still be required.