There's one major difference between private and password-protected help centers and this is their access control policy. With password-protected help centers, you can have an unlimited number of users view your help center. The only condition is they have the correct password. While with a private help center you should individually invite every viewer. If they don't have an account, they would sign up for a regular HelpCenter.io account and log in every time to be able to open your help center.
Password-protected help centers don't get limited by the number of users as they don't use any user accounts while private help centers are limited by the number of users you can get invited. The initial limit on our Startup plan is 10 user accounts per help center but you can always add more above that for some additional payment.
Password protected help centers don't get hidden from the world. They will still get displayed as an existing help center if somebody navigates to their address but they will prompt for a password first to get unlocked.
On the other side, private help center act like they doesn't exist unless you log in and have sufficient permissions to view it.
Something important to have in mind is, with a password-protected help center, you cannot have other than view permissions for a visitor logged in only with the password. However, you can still invite collaborators to your password-protected help center through a regular account.