The Scaled Print Size pane instead does act on the size of your graphic. You can change the size of your graphic by typing a percentage in the Scale field or by typing a value either in the Height or Width field. Changing value in either field will change the value of the other one proportionally. The little chain icon on the right factually means that the proportions will be maintained.
If the Show Bounding Box option is checked, Photoshop will show the boundaries of your graphic. In our example the black rectangle around the logo you see in the preview is the bounding box. You can see that the logo is significantly smaller than the page itself. The bounding box will not be printed with the image, it only shows up in the preview. It allows you to change the size of your graphic by dragging the mouse from it either inwards (to reduce the size) or outwards (to increase the size).
Under the Show Bounding Box option there is the Print Selected Area option. In our example it is grayed out. In order for that option to be available, you first need to make a selection then you can open the Print preview window by going to File>Print with Preview. The Print Selected Area option will then be available and if checked, Photoshop will only print the area inside your selection.