There are many kinds of image size units available. These are – percent, inches, centimeter, millimeter, points, etc. The output of each one of them is different than others. The clients need to be clear about the instructions given to us. Based on the instructions, we will deliver the images to them.
Learn more about Image Size Units:
Setting Default units for Image Size – Adobe Photoshop 7, CS, CS2 & CS3 by Justskins