  It may well happen that the two main components of the dark sector of the
Universe, dark matter and dark energy, do not evolve separately but interact
nongravitationally with one another. However, given our current lack of
knowledge on the microscopic nature of these two components there is no clear
theoretical path to determine their interaction. Yet, over the years,
phenomenological interaction terms have been proposed on mathematical
simplicity and heuristic arguments. In this paper, based on the likely
evolution of the ratio between the energy densities of these dark components,
we lay down reasonable criteria to obtain phenomenological, useful, expressions
of the said term independent of any gravity theory. We illustrate this with
different proposals which seem compatible with the known evolution of the
Universe at the background level. Likewise, we show that two possible
degeneracies with noninteracting models are only apparent as they can be
readily broken at the background level. Further, we analyze some interaction
terms that appear in the literature.
