If you're using the Passcreator API to create sub-accounts the Single Sign On feature is a convenient way to let your customers use Passcreator without having to log in manually or even knowing their credentials.
The process in general is that you request a SSO-Token using the API Token of the customer that is valid for one hour and can be used once. You will then use this token to log in the customer using HTTP Basic Auth. This request must be done within the browser of the user, usually using Ajax.
After the login has been successful you can redirect the user to the appropriate page within Passcreator.