You can find the 'MRZ_TestQuality: Strings' value in the response that is empty (as shown in the example below)
Actually, the "Strings": " is the array verification for each line in the result. It is provided when using Regula document reader devices/dedicated devices for a document authentication that performs the verification under the control environment with control of lighting, distance, resolution, etc.
It controls the consistency of ink in different light spectrums and provides the verification result. So, its actual usage is very limited (for example, it may be useful to control the MRZ printing quality for printing houses that actually issue documents).
In the context of Web API where the image is processed only in the white light, this verification result is not applicable.