Guam does not seem to filter groupware folders using their type definitions, but instead filters folders by name and prefix.
The intention is to filter folders based on their shared and private namespace values for the /vendor/kolab/folder-type value (anything empty or that does not start with 'mail' would be considered a groupware folder).
The blacklist strings are defined here: https://git.kolab.org/diffusion/G/browse/master/apps/kolab_guam/src/rules/kolab_guam_rule_filter_groupware.erl;28f63133a54dcccf8cefe4b1cf56a9f9e5d9301b$24