It depends on the issue with the skin, some are for a skin that may result in "hot spots" from a dog itching from allergies, so a medicated or antibacterial shampoo is given to heal the skin and reduce infection. Some are given if there is an actual skin infection or yeast infection going on with the skin. There are shampoos to dry skin out that is oily or weepy, there are ones to moisturuze the skin, etc. There are so many different types of RX shampoo's not to mention everything under the sun that pet store wants to sell you. Is there a certain shampoo that you are asking about one that maybe the Vet is recommending?
Here are some of the common vet prescribed shampoos
Pyoben (Medicated antimicrobial, keratolytic and follicular flushing shampoo formulated for the topical treatment of infection) ,
Dermachlor ( for dermatological conditions where antiseptic, antimicrobial, antifungal, cleansing) ,
Allergroom ( a moisturizing shampoo for allergic dermatitis)
Ecto Soothe ( natural oat-grain derivative, soap-free shampoo with hydrating and soothing properties),
Hexadene ( is an medicated, antimicrobial, antiseptic, cleansing shampoo ),
Ketochlor (is an antifungal, antibacterial shampoo to be used in the management of fungal skin conditions and bacterial dermatitis)
Ketoseb ( is a antifungal, antibacterial shampooto manage a variety of skin conditions, bacterial dermatitis, ringworm and malassezia [yeast] skin infections)
Sebolux (Antiseborrheic, keratoplastic and antiseptic shampoo that removes scales and crusts associated with seborrhea and other non-specific dermatoses)
Sorry if I over explained what you may have asked.... 23yrs in the Veterinary hospital and you tend to give more information than needed sometimes