Alert contact:

An alert contact can receive down/up/trouble alerts via email, phone, SMS and push notifications based on the configurations. The user will have no login permissions to Site24x7. 

Sub-user:

Sub-users, also called as multiple login users, can login to Site24x7 account and will have read/write access based on roles assigned. Super Admin and Admins will have complete access across the Site24x7 console. Sub-users can also receive alerts like an alert contact. A sub-user needs to be a part of the same organization account as that of the primary user. Know how to add a sub-user.