DolbyDigital Mix

[Vuts]

hey

i m a bit confused with this term? is it like a special way to mix for Dolby reference or is it just the volume level....
i know its AC3 right? and its 5.1 so 6 channels but what is the difference in two mixes when one is DDigital and the other is not!

any good reference links pls?

thanks guys!