With HDRx Settings set to Blend Exposures, this field behaves like the blend in REDCINE-X; the blend also attenuates the artifacts created by the scene operation, which is a blend of the two exposures: -1 shows only the Highlight (X frame), 1 only the Primary (A Frame), and 0 a 50-50 mix.
With HDRx Settings set to High Dynamic Range, Blend also attenuates the artifacts created by scene motions; set to 1 unless you are troubleshooting motion artifacts.
Setting the Highlight Threshold in HDRx Clips
RED import options such as ISO, FLUT, Exposure, and white balance change pixel values, and requires the Highlight Threshold to be adjusted accordingly.
To tweaking the Highlight Threshold in the Colour Corrector: