A low-impact attack indicates that the host is not vulnerable to those attacks. A low-impact attack is an attack that does not exploit any known vulnerability on the target host or does not match any signature or anomaly rule on the FTD device5. A low-impact attack does not mean that the attack is not dangerous to the network or that the host is not within the administrator's environment. It simply means that the attack did not succeed in compromising or affecting the host.
The other options are incorrect because:
* All attacks are not listed as low until manually categorized. The FTD device automatically assigns an impact level to each attack based on various factors, such as vulnerability information, threat score, and confidence rating5. The impact level can be high, medium, or low, depending on how likely and how severe the attack is.
* The attacks are not necessarily harmless to the network. A low-impact attack may still cause some damage or disruption to the network, such as consuming bandwidth, generating noise, or distracting attention from other attacks6. A low-impact attack may also indicate that the attacker is probing or scanning the network for potential vulnerabilities or weaknesses7.
* The host is not necessarily outside the administrator's environment. A low-impact attack can target any host on the network, regardless of its location or ownership. A low-impact attack does not imply that the host is external or irrelevant to the administrator's environment.

Reference:
https://www.cisco.com/c/en/us/td/docs/security/firepower/660/fdm/fptd-fdm-config-guide-660/fptd-fdm-routing.html

Explanation
https://www.cisco.com/c/en/us/td/docs/security/firepower/640/configuration/guide/fpmc-config-guide-v64/transp