The default web root location for MODX Cloud instances is /www/.
Some application frameworks recommend storing their files outside of web root. To accomplish this and to have an entire project included in backups, start by first accessing your instance via SFTP or SSH and create a directory for your alternative web root on the filesystem inside the /www/ directory.
Next, enter this same directory on the bottom of the Web Server tab of the Cloud Edit page. There is a single text input to specify a directory inside /www/ that will be used by NGINX as its web root. Once you save this value NGINX will then use it as the alternate web root location.