All system users created by you have SSH/SFTP access.
In order to ensure user level isolation, each SSH user would have a port assigned to that particular user. You would need to use that port to SSH/SFTP into the VPS. The assigned SSH port is shown with the N99panel console.
But there maybe a scenario whereby you can only access port 22 (the common port for SSH). In order to work with it, from within the N99panel console you can map the port 22 to a particular user. At any given time only one user can be mapped to port 22. For e.g. if userA has been mapped to port 22, when you map userB, the mapping of SSH port 22 will be removed from userA, and the new mapping with be then with userB.