  Recently, charge density wave (CDW) order in the CuO$_2$ planes of underdoped
YBa$_2$Cu$_3$O$_{6+\delta}$ was detected using resonant soft x-ray scattering.
An important question remains: is the chain layer responsible for this charge
ordering? Here, we explore the energy and polarization dependence of the
resonant scattering intensity in a detwinned sample of YBa$_2$Cu$_3$O$_{6.75}$
with ortho-III oxygen ordering in the chain layer. We show that the ortho-III
CDW order in the chains is distinct from the CDW order in the planes. The
ortho-III structure gives rise to a commensurate superlattice reflection at
$Q$=[0.33 0 $L$] whose energy and polarization dependence agrees with
expectations for oxygen ordering and a spatial modulation of the Cu valence in
the chains. Incommensurate peaks at [0.30 0 $L$] and [0 0.30 $L$] from the CDW
order in the planes are shown to be distinct in $Q$ as well as their
temperature, energy, and polarization dependence, and are thus unrelated to the
structure of the chain layer. Moreover, the energy dependence of the CDW order
in the planes is shown to result from a spatial modulation of energies of the
Cu 2$p$ to 3$d_{x^2-y^2}$ transition, similar to stripe-ordered 214 cuprates.
