By alternate mix we mean any variation from the original main mix. A classic example would be submitting for mastering both the full and the instrumental/PA mix.
This is very common for artists who might have to perform live using a backing track and want to make sure the outcome of their performance won’t be compromised by a poor sounding mix.
Another reason could occur when a radio station requests a “clean” mix without profanities to be passed on their playlists.
At Swift Mastering we do most of our work using analogue gear. This means everything needs to be done in real time with the machines being perfectly calibrated. Sometimes, tweaks for the instrumental version are needed too.
Needless to say that if a recall is needed all of the above has to be performed twice.
All this contributes to extra time and work for the ME.
This is a simple standard practice adopted by all mastering studios around the world.