Sub-accounts are very useful when you want to segment your application. For instance: a sub-account for IT, one for Finance and one for support. The advantage is that each department will have it's own contactlist, it's own reporting, etc ...
To get a list of sub-accounts that are active for your account at the moment, you can open the Sub-Accounts screen in the main menu. It will look like this:
You cannot create sub-accounts yourself. To add a new sub-account, contact us at firstname.lastname@example.org