why is CDD50 used in ashrae90

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Hi, everyone

A question was confused me for a long time, so have to the email to ask you, which is why CDD50 is used in Ashrae 90, CDD50 is defined as following:

cooling degree-day base 50°F (CDD50): for any one day,when the mean temperature is more than 50°F, there are as many degree-days as degrees Fahrenheit temperature difference between the mean temperature for the day and 50°F. Annual cooling degree-days (CDDs) are the sum of the degree-days over a calendar year.

However, I am very confused why 50°F is used as the threshlod between cooling and heating? That means when the outdoor air temperature is more then 50F, the cooling starts to work?In chinese standard, 26℃ is as the base of CDD.