Variable energy is a feature that some AED models have.
Energy is required to generate an electrical current. When a casualty is in cardiac arrest, most likely the casualty's heart is beating in an abnormal (or chaotic) rhythm. The electrical current from the AED will stop chaotic rhythm and allow the heart to re-start a new regular rhythm. A person's body mass will provide resistance against the movement of the electrical current. A person with a larger body mass will provide more resistance compared to a person with a smaller body mass.
An AED with variable energy will be able to to send more energy to create a current large enough to over come resistance of a casualty with large resistance.
Fixed energy is opposite to variable energy. AED models with fixed energy only have one energy level regardless of a casualty's resistance.
AED models that have variable energy also have escalating energy (the energy level increases with each subsequent shock).
Cardiac Science AED models have escalating and variable resistance. The first shock will be at a low range of energy levels; the second shock will have a mid range of energy levels; and the third (and any other subsequent) shock will have a high range of energy levels.