Thanks @Alan_Chan!
It looks like the explanation is in the QZV:
Feature list: The list of "core" feature IDs and their total percentile frequencies.
So the values in these TSVs are the percentile frequencies of those core features across all samples (the %s here are percentiles, not % samples threshold for defining core features, as in the visualization itself).
For example, D_0__Bacteria;D_1__Planctomycetes;D_2__Phycisphaerae;D_3__Tepidisphaerales;D_4__WD2101 soil group;D_5__uncultured bacterium;D_6__uncultured has a median frequency of 40.0
I hope that helps clarify!