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The difference between Idle and Session is network activity.
For example, if you set the session timeout for I minutes, after I minutes the NPS policy will cut the connection.
The idle timeout, based on my knowledge, if the connection is cut down for some network or other reasons, the NPS will hold this connection until the idle timeout. During this period, when your network recover, your client can connect automatically without NPS authentication again.
You can refer the following link for more information:
https://community.spiceworks.com/topic/1729351-rras-vpn-idle-timeout-not-disconnecting-users
Please Note: Since the website is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
Best Regards,
Candy