The Extract Alpha tool lets you extract the alpha channel from its primary input and has no control parameters. This tool always outputs an alpha-only image, implying that if the input is an RGB-only image, the output will be a constant 1.0 alpha image.
The Extract tool will extract a selected channel into an alpha-only image. The Extract Alpha tool is just a more convenient and readable tool to use when the selected channel is always the input alpha channel.
