Base temperature of heating and cooling degree days

Hello,
I have a question about the base temperature when we calculate heating and cooling degrees, it is set by default to be 18.3 °C, 23.3 °C for heating and cooling respectively.
Since ASHRAE 2006 set different figures10 °C, 18.3 °C,
What is the reference for these numbers in Ladybug?

Thanks in advance!