The reference directions of the voltage and of the current are the ones from figure 8.
Figure 8: Signs convention as source
With the reference direction chosen for the voltage, a voltage 
that is positive means that the potential 
of the B terminal is superior to the potential 
of the A terminal. With the reference direction chosen for the current, a current 
that is positive corresponds to the movement of the positive loads from A to B. The potential of the input terminal being inferior to the one of the output terminal of the current, the dipole provides therefore electrical power. The product 
is therefore positive.
If is positive and is negative, the potential of the input terminal of the current is superior to the one of the output terminal. The dipole absorbs electrical power. The product is negative.
If is negative and is positive, the load transfer is made from the terminal with higher potential to the terminal with lower potential. The dipole absorbs electrical energy. The product is negative.
Finally, if both and are negative, the load transfer is made from the terminal with lower potential to the terminal with higher potential. The dipole provides electrical power. The product is positive.
With the sign convention from figure 8, a product 
that is positive corresponds to a dipole that provides electrical power. A product that is negative corresponds to a dipole that absorbs electrical power. It is said to be the signs convention as source. This convention is usually chosen for all the dipoles planned to provide electrical energy (voltage and current sources, the batteries and other electric generators).
