The option you choose in this tab determines which area of the image becomes transparent:
None Produces a GIF file that does not contain one color set to transparency.
Existing Image or Layer Transparency Uses the current transparency information from the image.
Inside the Current Selection For images with a selection, this option makes everything within the selection transparent.
Outside the Current Selection For images with a selection, this option makes everything outside the selection transparent.
Areas that Match this Color Click the color box to select any color. Or, to choose a color from the image, move the cursor over the image, and click the color. In the Tolerance edit box, set a tolerance factor that indicates how closely colors must match the selected color before they become transparent.