What does the dirty flag mean in a FortiGate session configured for NGFW policy mode?

What does the dirty flag mean in a FortiGate session configured for NGFW policy mode?
A . The existing session table entry has been updated with the app_id and the firewall policy table needs to be checked for a match.
B . The application or URL category is unknown and needs to be rescanned by the IPS engine to try to identify the Layer 7 details.
C . The URL category for this session has been updated by FortiGuard and the session needs to be checked against the policy again to ensure proper web filtering is applied.
D . Traffic has been identified as coming from an application that is not allowed and the relevant replacement message needs to be displayed to the user, if configured.

Answer: A

Explanation:

Enterprise_Firewall_7.0_Study_Guide-Online.pdf p 99