The PromptOnSessionExpire setting takes effect if EnableSessionExpiration is set to true.
If both PromptOnSessionExpire and EnableSessionExpiration are set to true, a user whose session is about to expire will be presented with a warning prompt. This prompt gives the user 30 seconds to choose to stay logged in. If the user does not respond, the user is logged out.
If PromptOnSessionExpire is set to false while EnableSessionExpiration is set to true, no warning prompt is displayed when a user's session expires. The user is logged out automatically without prompting.