Hi, thank you for the reply but I am not sure I uderstand correctly. All I need is every user who will be a subscriber, he can be an administrator of his subsite and have no access to other subsites. I will split it in steps.

MAIN SITE: Sells subscriptions (BASIC & ADVANCED)
USER A: Buys subscription BASIC (gets the BUSINESS MANAGER role)
USER B: Buys subscription ADVANCED (gets the BUSINESS MANAGER role)
At the moment users are just subscribers with the BUSINESS MANAGER role and can edit their custom page in the main site.
MAIN SITE: Creates a SITE A and makes USER A the administrator of SITE A (additional user role BASIC MANAGER)
MAIN SITE: Creates a SITE B and makes USER B the administrator of SITE B (additional user role ADVANCED MANAGER)
In conclusion, both users have the BUSINESS MANAGER ROLE and their extra role and both are administrators on each sub site. Can USER A login to SITE B? In the users list of each subsite there are only the super admin and each administrator, USER A on SITE A and USER B on SITE B. Since USER A is not listed in the SITE B user list, is it possible to login with its credentials? I don’t want that. Thanks, I hope it is not complicated.