The Arbitrary control will allow you to type in the angle you wish to tilt the image or selected portion, either clockwise or anti-clockwise. Holding the shift key down while rotating will move the image in 15 degree increments. They are meant to be free-style commands. The Scale command will let you manually adjust the size of the image. For example, you can change a square into a diamond of equal size sides, but you could not change a square into a triangle.
Adjustment is only along the x and y axis individually.
You cannot adjust both at once. The Perspective command allows you to adjust the selected image, by shrinking any of the four sides. This can be very useful in making 3D looking objects, such as cubes, buildings, et cetera, which should look as if they are tapering off into the distance. Note that the nature of this command is such that all changes are made in a quadrilateral configuration. The Distort command works like a combination of all of the above.