In the Global Settings, on the Unit Settings tab, you specify length and angle units in two categories. One category is for normal use, such as for distance or angle measurement. The second category is for small values, such as residuals or standard deviations. These values are more clearly to read, if they are displayed in a smaller unit.
For example specify meter as distance measurement unit, and mm as unit for the distance standard deviation.
Large units, category 1, apply to
Small units, category 2, apply to