https://s3.amazonaws.com/s3.supplyho..._PROD_FILE.pdf
There are some details to study, basically outdoor reset makes the heating water hotter as the temperature outside goes lower.
4 dip switches set up temperature limits & a rotary sets the ratio.
In example, with outdoor reset ratio rotary set to 1.0
When the outdoor temperature drops by say 20* then the system water is increased by 20*
A well insulated house may use a .4 ratio & a poorly insulated house may need 1.4 ratio
You start with an educated guess & then fine tune by trial & error