There are some tiny little doubts for a newbie which usually appear and we don't really have the answer.
We know that sometimes we two ways to implement a process, and it works for the two ways, but we not always understand why.
This was one of my doubts, what was the great difference between a Business System and a Business Service...?
Of course I had a tiny little idea but now I have the answer and maybe it will be useful to anyone who questioned about it...
A Business System is defined in the SLD and is used as sender and receiver in the ID. However in some cases, the details of the implementing application are not available(in a B2B communication, where the system belongs to a different company. A Business Service represents an abstract entity for adressing the senders and receivers of messages. Therefore, when using Business Servces, it is no longer necessary to define any Business Systems in the SLD.