This read-only bitmap reports which link speeds, 10M, 100M, 1G, or 10G, on this interface's PHY actually support Energy Efficient Ethernet, since EEE support can vary by negotiated speed even on the same port. An administrator checks this before assuming EEE will take effect at a given link speed, since enabling EEE administratively doesn't guarantee hardware support at every speed. For example, if this bitmap shows EEE support only at 1G but the port is currently negotiated at 100M, the admin knows EEE won't actually be active at the current speed regardless of the admin setting.