ok, i'm sure this is a dumb question, but it's been bothering me for a while and I haven't figured it out.

Sometimes the yellow square in the middle of my transform tool stays flat to the camera - as in the picture on the left. At other times, the square stays flat to the world coordinates as in the picture to the right.

Not sure what I am doing to change it and changing the tool to world or object or whatever, seems to have no effect.

help! let me know if i haven't explained this very well.