With sub accounts you can control multiple accounts under your primary Relevant Tools account. Only the primary account has the billing information. Billing is aggregated for all the sub accounts.
For regional operations you can retain central control and oversight and enable regional staff to see only the databases that they are authorized for. Staff members can also be resticted in functionality so that they might not be able to delete or export databases.
For web developers or marketing agencies sub accounts can be used for each customer or client. Each sub account has their own unsubscribe lists and email footers. Billing is aggregated so you may charge for your services as desired.