  We review a recent letter published in Phys. Rev. Lett. $\textbf{116}$,
161804 (2016) of which the main argument is that the mode dependent magnetic
form factors at axion haloscopes depend on the position of the cavity inside
the solenoid while the corresponding electric form factors do not. We, however,
find no such dependence, which is also equivalent to the statement that the
electric and corresponding magnetic energy stored in the cavity modes are the
same regardless of the position of the cavity inside the solenoid. Furthermore,
we extend the statement to the cases satisfying $\vec{\nabla}\times\vec{B}_{\rm
external}=0$, where $\vec{B}_{\rm external}$ is a static magnetic field
provided by a magnet at an axion haloscope. Two typical magnets, solenoidal and
toroidal, satisfy $\vec{\nabla}\times\vec{B}_{\rm external}=0$, thus the
electric and corresponding magnetic energy stored in the cavity modes are
always the same in both cases. The energy, however, is independent of the
position of the cavity in axion haloscopes with a solenoid, but depends on
those with a toroidal magnet.
