[otrs] Documentation on Services and Service Level Agreements?

Hi, I am looking for documentation for "Services" and "Service Level Agrements". They are located under "Ticket Settings", as can be seen in http://doc.otrs.org/3.1/en/html/adminarea.html#adminarea-general I have browsed the admin manual, but could not find any hint on what a "Service" or SLA is good for (I have an idea based on common usage of these words, but that did not quite work out in my install). Could someone please point me at documentation on these topics? If it really does not exist, could someone give a short description? Cheers, Isaac