You can also use pixel values in the height and width field of the options bar by typing "px" after the numbers. For instance, if you have a web site and you like to post all your images at the same size of 400 x 300 pixels, you can create a preset for this size. When you use pixel values in the height and width fields, your image will always be resampled to match the exact dimensions.
The "Front Image" button on the the options bar comes into play if you ever need to crop one image based on the exact values of another image. When you click this button, the height, width, and resolution fields will fill in automatically using the values of the active document. Then you can switch to another document and crop to these same values, or create a crop tool preset based on the active document size and resolution.
