Superficial - mild acids are used in low concentrations, affecting only the top layer of skin. It is used for hyperpigmentation and to "refresh" the skin (more hydrated, brighter).
Medium - more concentrated acids are used, which affect more layers of the skin. Used for actinic keratoses, fine wrinkles, photoaging, scars.
Deep - uses acids in high concentrations, penetrates down to the dermis. It is used in the treatment of scars, but - because of the long recovery period and risk of side effects - it is rarely used.