The Transform Scaler sets the scale of the transform according to the screen resolution.
Scaling UI is not a good Idea. It will mess up the sizes and anchors of its children which makes it hard to control the positioning of the children for all resolutions.
As a rule of thumb you should only consider to scale UI elements which do not have any children and where the min and max anchor points are at the same location.
But in the very most cases you can achieve the same goal by adjusting the Size Delta of the object rather than the scale. So, usually you can (and should) use the Size Delta Sizer rather than the Transform Scaler.