Sub accounts can be used when there are separate entities in a company where an admin wants to access both accounts, but dont want the sub-account members to be able to access/edit each other's account.
From the Account Management page, click on 'Create New Account'.
This will then popup a window where you can enter the name of your new account.
This will open the new account in the manager, where you can then go to the parent account drop down and select your main account to link to.
You can confirm this has worked by going to the main account and looking at the list of Child/Sub Accounts.
You can now invite users to those sub accounts separately so they will have access to those accounts but not each other's accounts.