This command is found in
.This command reduces the size of a selection area by an arbitrary number of pixels either measured from the inside of a selection or the border.
Options
Pixel is default unit, but you can change it by clicking on the arrow button. Enter the shrink amount into the text box or by mean of the arrow buttons.
When this option is unchecked the borders of the selection who are over a border of the image are not shrinked.