Once all of your tracks have been cut it’s time to mix. This is when you combine all of your cut tracks—dialog, music, and effects—into one composite mix. In practice, the tracks are sub-mixed into pre-dubs that contain these 3 food groups. These pre-dubs are then used to create the final mix stems. The final mix or Print Master (for film) is created from these stems.
The reasoning behind stems is that it is easy to make changes to one group without affecting the other groups. For example, a person might use music that they are later unable to obtain the rights to. Since the music is mixed separately from the dialog and effects, it’s an easy task to drop in new music without having to re-mix everything else. If you want a foreign release, you’ll also have to provide an M&E. This is an additional expense above and beyond the budget of the domestic mix.