Citadel is distributed under the terms of the GNU General Public License (GPL), version 3.
Licensing Citadel under the GPLv3 guarantees that the entire software stack is 100% open source and will remain fully open source forever. Anyone is free to run, inspect, modify, and redistribute the code without restrictions.
Because of this guarantee, there will never be a crippled "community edition" versus a paid "pro edition." Many commercial vendors withhold essential features behind paid licenses or proprietary add-ons. In contrast, every feature Citadel offers is included directly in the main codebase, freely available to everyone.