The Transparency Fill Tool erases pixels that have roughly or exactly the same color or brightness as the clicked pixel in a flood-fill fashion. The tolerance level is controlled by the level setting and the amount of transparency applied is controlled by the alpha #2 setting in the toolbox. This tool can be used to remove
backgrounds, as is being done with the photo of the stranger below. Consider using the Shrink Alpha effect to clean the edges.
Options
Normal
This option uses all the color channels to decide what to erase. Pixels that roughly match the pixel under the cursor are erased or partially erased depending on how much they match.
Alternate
This option uses all the color channels to decide what to erase just like the normal mode, but using a different algorithm. Pixels that roughly match the pixel under the cursor are erased or partially erased depending on how much they match.
Chroma
This option uses the chrominance (color) to decide what to erase. Pixels that roughly match the pixel under the cursor are erased or partially erased depending on how much they match.
Luma
This option uses the luminance (brightness) to decide what to erase. Pixels that roughly match the pixel under the cursor are erased or partially erased depending on how much they match.
Exact
This option uses all the color channels to decide what to erase. Pixels that roughly match the pixel under the cursor are erased or partially erased depending on how much they match.
The Transparency Fill Tool erases pixels that have roughly or exactly the same color or brightness as the clicked pixel in a flood-fill fashion. The tolerance level is controlled by the level setting and the amount of transparency applied is controlled by the alpha #2 setting in the toolbox. This tool can be used to remove
backgrounds, as is being done with the photo of the stranger below. Consider using the 
Normal
Alternate
Chroma
Luma
Exact