The phrase "additional sweetener" means something that is added to make things more pleasant or appealing. It is often used metaphorically to describe an extra benefit or advantage that is given to make a situation better. Full definition
Process until the mixture is smooth and creamy, adding additional sweeteners if desired. (insonnetskitchen.com)
If I left out the protein powder, would I need additional sweetener? (thehealthyfoodie.com)
The natural sugar from the apples and banana, as well as some from the coconut milk, leave this smoothie tasting sweet without the need for additional sweeteners. (mysanfranciscokitchen.com)