Auto-Rig Pro is released under the terms of multiple licenses:
The add-on source code components of Auto-Rig Pro that include the BPY module are released under the terms of the GNU General Public License, version 3.
Assets files such as icon image components, and rig components in .blend format, are released under the terms of the Royalty Free license and CC0.
Refer to the LICENSE file in the subfolders for more details.
In any case, a rig generated with Auto-Rig Pro by the end-user (such as in the process of rigging a character), is the sole property of the end-user who has full rights on it.