The document discusses the application of the Biopharmaceutics Classification System (BCS) in formulation development. It describes the four BCS classes based on a drug's solubility and permeability properties and how these properties impact bioequivalence studies and regulatory requirements. It also discusses factors that influence the predictability of in vivo performance based on the BCS class, such as first-pass metabolism and excipient effects. Recommendations are provided for designing successful bioequivalence studies using BCS principles.