Microsoft Defender AV must block abuse of exploited vulnerable signed drivers.
This policy setting prevents an application from writing a vulnerable signed driver to disk. Vulnerable signed drivers can be exploited by local applications that have sufficient privileges to gain access to the kernel. Vulnerable signed drivers enable attackers to disable or circumvent security solutions, eventually leading to system compromise.
The Block abuse of exploited vulnerable signed drivers rule does not block a driver already existing on the system from being loaded.
How this relates to the STIG checklist: STIG vs SCAP benchmark.