Serum is the sixth step in the routine.
Serums usually contain the most highly concentrated active ingredients compare to the rest of your routine. Due to the high concentration, it is recommended to use serum few times per week as they are designed to treat very specific skin concerns. They are also usually the most expensive due to the same reason. Compare to essence, serum is usually much thicker in its texture and usually comes in a small dropper bottle. The most common types of serums are:
Skin barrier strengthening
in the morning
It is completely fine to use different serums to target different problems in the morning and at night. Do remember to always use sunscreen before heading out if your day time routine includes a Vitamin C serum!