'When used in combination' matlab jab aap multiple actives ya ingredients ko ek saath use karte ho. Inke combined effects aur skin tolerability poori tarah se depend karti hai kis specific ingredients ko aap mix kar rahe ho. Kuch ingredients ek dusre ke saath accha kaam karti hain, lekin kuch ko separately use karna better hota hai skin irritation se bachne ke liye. So yeh phrase basically batata hai ki formula mein kaun se ingredients ek saath combine hain aur unka overall effect skin par kaise hota hai.