I understand the difference between Business System and a Business Service (as I can read sap help and the previous posts on this subject thankyou).
I have a 3rd Party Warehouse system (Manhattan).
I have defined this in the SLD and imported this into XI as a Business System. As that seemed like the correct way rather than simply just defining it as a Business Service.
I have used this in various configurations and it seems fine.
But check this out.
If in XI Configurator you go into the Business System it is not possible to attach Receiver and Sender Interfaces to it like you would for a Business Service. This means that everytime you want to use an Interface you have to do a search all instead of just being able to picking it up off the ones defined against the System (like you can for a Business Service).
This is a little annoying and I wonder if I am doing something wrong.