The work produced by the flowing of the charges through the element is mirrored by a difference between the potentials of the two terminals.
If the element absorbs the energy of one Joule (1 J), due to the flowing of the charge of one Coulomb (1 Cb) from A to B, this is because the voltage drop 
is of one volt, measured between A and B (the potential 
of the A terminal is 1 V greater than the potential 
of the B terminal).
Figure 4: The difference of the potentials, measured between the terminals A and B
If the element furnishes the electric energy of one Joule (1 J), due to the flowing of the charge of one Coulomb (1 Cb) from A to B, this is because the voltage drop is of one volt, measured between B and A (the potential of the A terminal is 1 V smaller than the potential of the B terminal)).
Figure 5: The difference of the potentials, measured between the terminals B and A
Concluding, when absorbs the energy, the potential of the input terminal for the current, is greater than the one of the output terminal. When the dipole furnishes the energy, the potential of the input terminal for the current is smaller than the one of the output terminal.
The difference of the potentials that exist between the two terminals of a dipole is also named voltage across the element or voltage drop. The reference sense of this voltage drop can be chosen arbitrarily: if the potential of the "+" terminal is really greater than the potential of the "-" terminal, the voltage drop is positive; if it is smaller, than the voltage drop is negative.
Figure 6: The voltage across a dipole
