The Add Margin image transform toolbar operator adds the specified number of pixels to the top and sides of an image. By default, these are added as black pixels by default. The Crop Margin command
Crop Margin was used to trim 20 pixels from the margin of the center image to create the image on the left. To create the image on the right, we've added 10 pixels to the margin of the central image. By default, the Add Margin operator adds colors in whatever color is currently set as the foreground color in the format toolbar for images. Un-checking the Fill new pixels with current color box in Tools - Options will cause the margin to be added using invisible pixels regardless of the current foreground color.
Note: When used with surfaces, the Add Margin transform always adds invisible pixels.
Use with Selections
Selections do not affect the Add Margin or Crop Margin commands since these operate on the overall image rectangle.