New User Registered: This will send a notification once a user has registered, and their account has been validated (either by email, or admin approval)
New User Awaiting Admin Approval: This will send a notification when a new user registers and they require admin approval
User's Email Becomes Invalid: If you use XenForo's email bounce handling this will send a notification if a user's email has bounced too many times and they've been required to revalidate their email
User Deleted: This will send a notification when a user's account has been deleted