Recommendation ITU-T G.663 covers application-related aspects of optical amplifier (OA) devices and subsystems, primarily those used in digital systems. Applications include both single-channel and multi-channel systems used in point-to-point and point-to-multipoint configurations for use in long-distance networks and optical access networks. The purpose of this Recommendation is to identify which aspects should be considered for each application and to specify appropriate parameter values and ranges for each type of OA device. In this version of this Recommendation, information about non-linear optical effects in Appendix II is updated.