Pixel Margin
Watch the following part of the tutorial for an introduction to the functionality:
The Pixel Margin functionality allows the margin of the resulting UV map to be determined in pixels of the active texture. In order to use this option, enable the check box located in the Pixel Margin subpanel header:
With the Pixel Margin functionality enabled, the Margin option from the Packing Options subpanel will be ignored and the packer will determine the correct pixel margin set by the user in the Pixel Margin parameter.