It is used as a voltage reference and is controlled by Precision.The handy Tube variable is used in this program.4 pins are included in the voltage reference ic.
LM399H#40057PBF Features
LM399H#40057PBF Applications
There are a lot of Analog Devices LM399H#40057PBF Voltage Reference applications.
Suitable for use with a wide range of video RAMDACs