A) The minimum amount of power needed to complete the contact
QRP (low power) operators already understand the meaning of this rule. They accomplish DX (long distance) contact with just a few watts or sometimes even miliwatts. It is rather fun to turn down the power during a contact to see "how low you can go".
This rule becomes even more critical with a space station since transmitting on the uplink at greater power than necessary increases the chances you will cause harmful interference.
Written by: NJ2X, Michael Maher