Speaking as a procedural ATCO, you give traffic information to any aircraft that APPEAR to be in the same area, or are self evidently conflicting with each other!! The "duty of care" is as good a defintion as any, and applies to non-radar ATSU's as well as radar units!!! (Sorry radar guys, joking really!!)
If in doubt, give information!!
Keep the faith!!
[ 07 November 2001: Message edited by: ProcATCO ]