A Passive Optical Network (PON) is a cost-effective network topology that addresses the last mile of the communication infrastructure connecting the central office (CO) to the business or residential customer. ITU-T recommendation G983.1 defines three classes of performance for APON (ATM-PON) systems. The three classes address different range requirements. This enables tradeoffs between cost and reach, and consequently operators can offer cost-effective solutions based on specific customer requirements. This note summarizes the requirements of various classes.