Microsoft Defender AV must be configured to process scanning when real-time protection is enabled.
This policy setting allows the configuration of process scanning when real-time protection is turned on. This helps to catch malware, which could start when real-time protection is turned off. If this setting is enabled or not configured, a process scan will be initiated when real-time protection is turned on. If this setting is disabled, a process scan will not be initiated when real-time protection is turned on.
How this relates to the STIG checklist: STIG vs SCAP benchmark.