What is a yellow pill with sl and 563 on it?

A yellow pill imprinted with "SL" and "563" is identified as Diphenhydramine Hydrochloride 25 mg, which is an over-the-counter antihistamine commonly used to relieve symptoms of allergy, hay fever, and the common cold. It can also be used as a sleep aid due to its sedative properties. Always consult a healthcare professional before taking any medication to ensure it is appropriate for your needs.