transmitted from terminal $i$ to terminal $j$, and $t_{ij}(E) f_j(E)$ is the same but from terminal $j$ to terminal $i$. Therefore, $I_i$ is the *net* average electric current at terminal $i$, counted positively from the terminal to the system.

[To describe the transport of neutral particles (such as neutrons or phonons), it suffices to set $e = 1$ in and one will obtain the particle current.]{.nodecor}

It turns out that most relations that we shall encounter will have the same form in the three considered cases (an example being given by the expression ). Therefore, from now on, all relations not wearing the superscript MB, FD or BE will be assumed to hold in all three cases.

We shall also obtain some expressions which do not depend at all on the distribution function describing the reservoirs. In order to emphasise the fact that such relations do not depend on the nature of the particles, we shall say that they are *universal*. We hope the reader will not be confused with the notion of universality used to refer to properties that are model independent.

Under the Assumption A\[Assumptions\], one can also derive the expression for the (average) *heat current* at terminal 