« Last post by hubearth on March 27, 2017, 11:48:05 AM »
I figure it out myself. I post the reply in case someone else would have the same question.
Nothing in the docs refers to this option explicitly. Looking in the database structure, in the table carddav_adressbooks, I found a field use_categories.
I tried adding 'use_categories' => true in the global settings of carddav plugins and bingo, it works.
Although, if the users already had the carddav server in their settings, you need to delete all references in the database before logging in.
So, I just deleted the plugin, droped all carddav tables, re-installed the plugin and logged in.
Now, it's perfect