For a general irreversible process, incorporating sub-processes, entropy change may be expressed as the vector product of a thermodynamic force- and flow vector. A fundamental hypothesis is introduced, whereby a general irreversible process or sub-process may be categorized as being of a non-residual type, or of a residual type. A non-residual irreversible process is typically associated with continuous, linear behavior, where true material properties represent model constants, and the principle of local equilibrium is valid. A residual irreversible process is typically associated with discrete-, non-continuous-, and often non-linear behavior, where model constants do not represent generally accepted material properties.