Base cleaner (red box) is for cleaning the grip wax zone. Glide wax cleaner (black box) is for cleaning the glide zone.
Swix recommends that you use a red box, called Base cleaner, when cleaning grip wax from the attachment zone on your skis.
The black cleaner, called Glide wax cleaner, is used to clean the glide zone of contamination. You should do this especially in the spring when the snow is "dirty".