s3tc compression

Could someone explain what the difference between GL_COMPRESSED_RGBA_S3TC_DXT3_EXT and
GL_COMPRESSED_RGBA_S3TC_DXT5_EXT is? Im having a bit of trouble understanding the extension document. Thanks.

DXT3 : uncompressed alpha, 4bit per pixel.

DXT5 : compressed alpha.
each 4x4 tile has 2 8bit values: alpha0 & alpha1,
and 3bit per pixel, which are:

if (alpha0 > alpha1)
8 levels of alpha from alpha0 to alpha1
6 levels from alpha0 to alpha1 and 2 additional const. values: 0 and 1.0 (=255)