System property view lists all available system properties that could be used in expression. You can click the "System Property" button on the right toolbar, or press Ctrl+K (Command+K in Mac OS) to show/hide the system property view.
The table lists the name, type and description of every system properties. If you select a system property in the table, the sub view on the bottom will show how it looks within expression. The system property is read only, and you will not able to change its value via action.