Final answer:
The EICAS status message AC ESS ALTN indicates that the essential bus has switched to an alternate power source, commonly AC bus 2, due to a deviation from its normal power supply.
Step-by-step explanation:
The EICAS status message AC ESS ALTN indicates that the essential bus is powered by an alternate source, which in the context of aircraft systems typically means that the AC essential bus is not receiving power from its primary source and has automatically switched to an alternate power source, such as AC bus 2. This message serves as a caution that the system has deviated from its normal power supply configuration. It is important for pilots to note this condition so they can take necessary actions according to the aircraft's operational procedures, which might include checking the status of generators, transformer rectifiers, and the power distribution panel to determine the cause and to restore normal operations.