It extends the DefineBitsJpeg2Tag class by including a separate zlib compressed table of alpha channel values. This allows the transparency of existing JPEG encoded images to be changed without re-encoding the original image.
Although the encoding table defines how the image is compressed it is not essential. If a DefineBitsJpeg3Tag object is created with an empty encoding table then the Flash Player will still display the JPEG image correctly. The empty encoding table is not a null object. It contains four bytes: 0xFF, 0xD9, 0xFF, 0xD8. Note however that this is reversed from StartOfImage (SOI, 0xFFD8) and EndOfImage (EOI, 0xFFD9) tags defined in the JPEG file format specification. This appears to be a bug in Flash. However the order is preserved to ensure compatibility although code has been tested with the normal order for the tags and the images were displayed correctly.
This tag was introduced in Flash 3.