  High-mass stars are formed within massive molecular clumps, where a large
number of stars form close together. The evolution of the clumps with different
masses and luminosities is mainly regulated by its high-mass stellar content
and the formation of such objects is still not well understood. In this work,
we characterise the mid-J CO emission in a statistical sample of 99 clumps
(Top100) selected from the ATLASGAL survey that are representative of the
Galactic proto-cluster population. High-spatial resolution APEX-CHAMP+ maps of
the CO(6-5) and CO(7-6) transitions were obtained and combined with additional
single-pointing APEX-FLASH+ spectra of the CO(4-3) line. We study the
correlations of the CO line luminosities and profiles for the three CO
transitions with the clump properties and investigate if and how they change as
a function of the evolution. All sources were detected above 3-$\sigma$ in all
three CO transitions and most of the sources exhibit broad CO emission likely
associated with molecular outflows. We found that the extension of the mid-J CO
emission is correlated with the size of the dust emission traced by the
Herschel-PACS 70 $\mu$m maps. The CO line luminosity is correlated with the
luminosity and mass of the clumps. However, it does not correlate with the L/M
ratio. The dependency of the CO luminosity with the properties of the clumps is
steeper for higher-J transitions. Our data seem to exclude that this trend is
biased by self-absorption features in the CO emission, but rather suggest that
different J transitions arise from different regions of the inner envelope.
Moreover, high-mass clumps show similar trends in CO luminosity as lower mass
clumps, but are systematically offset towards larger values, suggesting that
higher column density and/or temperature (of unresolved) CO emitters are found
inside high-mass clumps.
