A quick service credit is one entree and one beverage. (Now includes speciality drinks - slushies and those 21 and older alcohol - where available.)
At the resort quick service location, they are allowing guests to use their refillable mug for their drink and adding a side of dessert (substituting).
The mugs are not valid at the theme parks. It is at the discretion of the cast member\restaurant manager, as to whether a substitution is allowed.