It is a secondary thermometer and is calibrated against the International 2001 Provisional Low Temperature Scale (with 0.904 mK the magnetic ordering of solid He3 in zero field and 315 mK for the minimum of the melting curve).
Click on an image for a larger version and details.
R. L. Rusby, et al., The Provisional Low Temperature Scale from 0.9 mK to 1 K, PLTS-2000, J. Low Temp. Phys. 126, 633-642 (2002). Read online.