How to restrict access to the document root for FTP users so that each FTP user has access to a certain folder only and could not access the document root for a domain?
By default FTP home directory for a system user is document root for a domain:
This user (subscription owner) will have full access to all folders inside Domain > example.com > File Manager via FTP.
Additional FTP user's home directory can be specified during his creation in Home > Domains > example.com > FTP access > Add an FTP Account:
In that case additional FTP user will have access only to his Home directory.