The SB version has a single user account architecture more suited for single organizations or businesses. You can have multiple administrators with different roles but administrator accounts are not isolated. For example, if you have two Newsletter editors they can both see and edit the same newsletters.
The MB version is technically identical to SB but with a different licensing.
The ESP version allows you to create multiple, completely isolated and independent user accounts.
nuevoMailer SB is mostly suited for,
A single business or organization that does email marketing.
The organization may have different departments and each department may have its own mailing lists (and Sender profile settings).
A subscriber can of course be subscribed to several lists.
Different roles can be assigned to various administrators. Like, reports-only user, newsletter editor, list manager and more. But there is no data isolation among these administrators.
IT professionals that want to send campaigns on behalf of their clients.
You can use a different Sender profile (sender details, sending method, landing pages, etc) for each client.
You can share an encrypted link with your clients to access a campaign summary report.
But you may not give login access to a client (in case you manage lists from many different clients).