Sorry for late reply.
-Threshold=0 is an automatic calibration. The system checks the average noise level and set the level to 10% above that. If Threshold is set to 0 and there is no noise Threshold will be set to 1, but it is still in Automatic mode.
-Set Threshold manually to 1 is extremely low and the system will trigg for any group of pixels that is interpreted as moving. This can be noise or actually an object that moves.
What is best is always tricky and will depend on situation. If you know quite well what object you want to detect and about how much of the area it will cover then a manual set can be good to minimize false alarms. If it is difficult to say the automatic mode might be better.