Brendon,
1. When you create their user account (non administrator), you can assign them privileges to certain groups, policies, outbreak control lists, custom detection list, exclusion lists, etc. You can also go in and edit these as needed under Accounts > Users. If you create separate groups, policies, objects, etc. for your various customers they will only see data for their connectors.
2. If they make a subscription for one email per event, they will get an email for each event based on the subscription settings. This is typically used if they have an automatic ticket generating system based on each email. It will go to the email address listed as the notification email (Accounts > Users > click a user) for the user creating the subscription. If they only have privileges for some connectors, they will only get emails concerning those connectors.
If you have an Admin account and are monitoring all systems, you will get alerted when they have detections if you have subscriptions set up for all policies.
Thanks