Internal users use subscription to sign up for notifications themselves, but if you want to send notifications to external recipients (for example, business partners) it is not possible for them to use subscription in the same way. This is because they do not (normally, at least) have access to your Intranet, nor are they registered as Windows users in your company.
Therefore, external recipients must be explicitly added in the Notification Designer. This does mean that you hard code names of these recipients into the schemes. To send notifications to the relevant business partner when Navision has raised an event, the e-mail address to send the notification to must be part of the data that Navision sends (the business entity). For a customer, for example, there is a standard field in the Navision Customer table for this called "E-Mail."