Oh, I see. That makes sense. But when I think about it I wonder why would someone want to use a negative gauge factor over a positive one?
Depends on the configuration of your Wheatstone bridge for example.
Your previous topic showed two strain gauges of equal type on the bottom half of the bridge. This results in both sides of the bridge changing as the sensors react.
By using one positive and one negative, you could put both sensors on one side of the bridge or both! Imagine how sensitive you could make a bridge with 2 of each sensor. You would not use a resistor in the bridge.