Fit to width constrains the height and width to the setting you specify for the width.
Fit to height is the same except you specify the height.
Fit to box will allow you to specify unique dimensions for the height and width.
If you select fit to width or fit to height and enter 100 pixels, both the width and height will be set to 100 pixels.
using fit to box, you can specify that the width should be 100 pixels and the height should be 150 pixels.