Cisco Firepower impact flags are indicators that help you evaluate the impact an intrusion has on your network by correlating intrusion data, network discovery data, and vulnerability information1. Impact flags are assigned to intrusion events based on the following criteria:
* The operating system and application protocol of the target host
* The exploitability of the target host by the attacker
* The relevance of the intrusion rule to the target host
* The severity of the intrusion rule Impact flags can have four values: unknown, neutral, affected, or vulnerable. Unknown means that the system does not have enough information to assess the impact.
Neutral means that the system knows the target host is not affected by the intrusion. Affected means that the system knows the target host is affected by the intrusion, but not necessarily exploitable. Vulnerable means that the system knows the target host is exploitable by the intrusion1. Impact flags can help you prioritize your response to intrusion events, as well as generate reports and alerts based on the impact level. You can also use impact flags to filter and search for intrusion events in the Firepower Management Center1. References: 1: Firepower Management Center Configuration Guide, Version 6.1 - External Alerting with Alert Responses.

https://www.cisco.com/c/dam/en/us/products/collateral/security/cognitive-threat-analytics/at-a-glance-c45-736555.pdf