Movement of a component within a system or across its boundary. It may be expressed using different kinds of quantities, e.g rates of change \(\frac{\rm{d}Q}{\rm{d}t}\) or \(\frac{\Delta Q}{\Delta t}\). Examples are: mass rate, \(\frac{\rm{d}m_{\rm{B}}}{\rm{d}t}\) or \(\frac{\Delta m_{\rm{B}}}{\Delta t}\); substance rate, \(\frac{\rm{d}n_{\rm{B}}}{\rm{d}t}\) or \(\frac{\Delta n_{\rm{B}}}{\Delta t}\).