An exchange law modifies one or more quality classes, based on parameters specific to the law, hydraulic variables, and possibly concentrations of some quality classes.

Thus, the exchange terms relative to law $i$ are calculated as follows:

$$\left(E_{k_1}^i, \dots, E_{k_n}^i\right)=L^i\left(C_{k_1}, \dots, C_{k_n}, p_1, \dots, V, h, \dots\right)$$

The exchange term of a class $k$ is the sum of its exchange term in every law where this class is present :

$$ E_k=E_{k}^{i_1}+E_{k}^{i_2}+\cdots+E_{k}^{i_n}$$