I don't quite understand what you are trying to achieve but the xyz + and - determines how an object points at the Target Helper Object. If you are using a symmetrical object like a plane then this may not be apparent but if you use an irregular object you'll see choosing x+ or z- etc. changes the side that faces the THO. It may be that you'd be better off moving the hotpoint of your plane to the camera and numerically rotating it using the Properties tray.