I recently purchased CC to organise my manuals and reference books (mostly PDFs)
As User Categories don't seem to be present in CC I followed the instructions here:
http://manual.calibre-ebook.com/sub_groups.html
to create a group/custom column called Manuals with subgroups like Photography, Tools, Kitchen, IT.
This works fine in Calibre (Windows). If I click on my Manuals group at the top level I see all my manuals and for a sub-group the relevant subset.
In CC however
when I select my Manuals group CC actually displays all my books not just my manuals 
(the subgroups are fine).
My custom metadata field for manuals against the other (non manual) books is empty, so they definitely shouldn't be in the Manuals grouping.
Is this a bug, or am I doing something wrong? Note: "All #manual" is shown in the top left corner when I select my Manuals group.