Here you can add/edit/delete Systems.
The Manage Systems form allows you to add/edit/delete Systems and their descriptions.
Here you can create a list of Systems to be used within Projects. Each Item that you add to the Project can be assigned a System to help you stay organized.
When you want to add a list of Systems to a Project.
You can add/edit/delete Systems.
Many of the reports in SI 2015 have Report Definitions set to group by System so it is a handy field to assign to Items within a Project.