For example, 10GBASE-KR is a 10 Gbps (10G) data rate baseband (BASE) specification, with a backplane (K) medium, using a 64B/66B (R) coding scheme, in a single lane configuration. This is purely an electrical specification that fully defines the features and characteristics of a compliant Ethernet PHY.
10GBASE-KX4 is also a 10 Gbps baseband specification for a backplane; however, it uses 8B/10B (X) coding, in an aggregated 4-lane configuration. Even though both 10GBASE-KX4 and 10GBASE-KR are 10 Gbps electrical interfaces, they describe different PHYs. A 10GBASE-KX4 PHY operates at 1/4 rate of the 10GBASE-KR across 4 lanes to achieve the same throughput.
Similarly, although 10GBASE-ER is a 10 Gbps baseband specification, it is not an electrical description like 10GBASE-KR or 10GBASE-KX4. 10GBASE-ER is an extra-long reach (E) single mode optical transceiver specification that utilizes 64B/66B (R) coding, capable of 40 km fiber optic cable reach. 10GBASE-ER mainly describes the requirements of an optical transceiver and does not provide the electrical requirements of a PHY that could drive the transceiver.
Therefore, it is important to distinguish the differences between the optical transceiver specification and electrical specifications defined in the IEEE 802.3 standard.