The Relative Image Brightness (Luminosity) is adjusted to 55% which means that the filter background indicated by the largest peak (gray value with the highest intensity in pixel counts) appears in the middle of the gray value range:

Relative image brightness is adjusted to a Luminosity value of 55%. (Source: ISO 16232)
Relative image brightness is adjusted to a Luminosity value of 55%. (Source: ISO 16232)

The Relative Threshold is set at 70% referring to the Relative Image Brightness (Luminosity) setting.

Relative threshold is adjusted to a value of 70%. (Source: ISO 16232)
Relative threshold is adjusted to a value of 70%. (Source: ISO 16232)

For information on setting the relative image brightness (Luminosity), see Camera Tool and Light Path Editing Tool.

For information on relative threshold, see Particle Segmentation Tool (Components).