At Treatment stage, products used should contain key active ingredients that address to your major & specific skin concerns such as wrinkles, hyperpigmentation, blemishes and dull skin etc! Treatment stage includes 5 (Essence / ampoule), 6(Serum), 7(Mask), and 8(Eyecare).
Especially Essence, Ampoule, and Serum should be used based on their consistency, always start from the lightest to thickest to maximize absorption efficiency. Normally it is Essence > Serum> Ampoule, but it can be different by each brand.
It is not necessary to use Essence, Ampoule, and Serum at one go. However, it is still important to find 1- 3 right products for you! It can be 1 almighty ampoule for you. Or it can be essence and serum combination for you. Or you might need to use them all three if your skin is super dry!
Find your magic treatment combination to address your major skin concern and use them regularly depending on what your skin type and condition requires on that day.