What does the 'X' stand for in the informal IEEE Standard Name, such as 1000BASE-LX?

In the context of informal IEEE Standard Names like 1000BASE-LX, the 'X' denotes the type of medium used for data transmission. Specifically, 'X' indicates that the standard is designed for fiber optics. This means that the specifications outlined under this nomenclature pertain to the characteristics of fiber optic technology, which is more commonly used for high-speed networking situations due to its advantages in bandwidth and distance compared to copper.

While other choices reference different types of media or signaling methods, the designation of 'X' is consistently associated with fiber in these IEEE standards. Therefore, identifying 'X' as representative of fiber makes clear the intended application and the physical medium used in conjunction with the other parts of the name, such as '1000BASE' indicating a gigabit Ethernet standard.