  The Poynting vector is an invaluable tool for analysing electromagnetic
problems. However, even a rigorous stress-energy tensor approach can still
leave us with the question: is it best defined as $\Vec{E} \cross \Vec{H}$ or
as $\Vec{D} \cross \Vec{B}$? Typical electromagnetic treatments provide yet
another perspective: they regard $\Vec{E} \cross \Vec{B}$ as the appropriate
definition, because $\Vec{E}$ and $\Vec{B}$ are taken to be the fundamental
electromagnetic fields. The astute reader will even notice the fourth possible
combination of fields: i.e. $\Vec{D} \cross \Vec{H}$. Faced with this diverse
selection, we have decided to treat each possible flux vector on its merits,
deriving its associated energy continuity equation but applying minimal
restrictions to the allowed host media. We then discuss each form, and how it
represents the response of the medium. Finally, we derive a propagation
equation for each flux vector using a directional fields approach; a useful
result which enables further interpretation of each flux and its interaction
with the medium.
