Error logger is a real-time report based on a specific criteria.
You can define the legal ranges for your signal and Error Logger will report any values that go outside of your pre-defined ranges.
The ranges can be configured in Help / QC Gamut Check:
Once the Gamut Check option is enabled and Error Logger session is started you will see every out-of-gamut situation logged in the list.
Gamut Check analyses the source frame without any modifications. It's in original resolution, before the crop or aspect ratio is applied.