In order to be their most effective, cosmetic ingredients need to penetrate the epidermal layers sufficiently to perform their specific function, and this will vary depending on the type of product you are creating.
For example, a moisture protective product only needs to penetrate (and preferably provide some sort of film forming) at the stratum corneum level; while a product designed to boost hydration over a prolonged period will need its ingredients to penetrate to the mid-layers of the epidermis whilst also preferably providing some moisture protective properties at the stratum corneum level.
Advanced cosmeceutical ingredients that activate dermal-epidermal communications, need to penetrate to the stratum basale level.
The key thing to remember when formulating is: where do you want the ingredients to go, and how are you going to get them there?
Read on & get further guidance!