The system is informational only - you don't get resolution advisories. But it does tell you about where other traffic is in both azimuth and relative altitude, so the resolution is obvious. In a head-on situation, you turn to the right, etc.
IO540
You prove my point for me that some people are operating under a misconception. There are people out there who will use their ACAS system (which includes TCAS and everything else in the market) in the horizontal plane. The system is not designed for that and is not accurate enough in providing bearings. They could be up to 30 degrees out.
Turning to the right simply because your display says the traffic is head on could actually be turning you right in to the guy you are 'blindly' trying to avoid. ACAS is only accurate for vertical avoidance manouevres, that is how it is designed.
It is not an airborne radar or an accurate horizontal situation display. It can give you a clue as to where to look and you might sight the traffic involved (or you might not). You can then take appropriate horizontal avoiding action. If you don't sight the traffic and manouevre horizontally based on the picture being given to you by your kit, don't be too surprised if you get a nasty surprise on occasion.