"Use System Value"
If you go to the admin client and open the tab where you see the three values and then go to the HELP you'll see:
Use System Value
This option is available only when adding or updating a user. Use the time out value from the Login Information page of the System Parameters window.
This refers back to the System Parameters for the instance, the Login Information tab.
On that page there are two possible selections, either Never Time Out or Time Out in nnn Minutes
That's where for that user, if that option is selected, the timeout value will be derived.
Ed