The Zone Cooling Design Supply Air Temperature is the temperature of the air that you are supplying to a zone in order to meet the cooling setpoint. It has nothing to do with the DDY file. The DDY file is the External Cooling and Heating Design Day conditions which define the external environmental conditions to be used when sizing the HVAC systems.
Suppose you are modeling a 90.1 baseline system 7, you should follow appendix g 18.104.22.168
G22.214.171.124 Design Airflow Rates. System design supply
airflow rates for the baseline building design shall be based on
a supply-air-to-room-air temperature difference of 20Â°F or the
required ventilation air or makeup air, whichever is greater. If
return or relief fans are specified in the proposed design, the
baseline building design shall also be modeled with fans serving
the same functions and sized for the baseline system supply
fan air quantity less the minimum outdoor air, or 90% of
the supply fan air quantity, whichever is larger.
This is saying that if you are cooling to a zone air temperature setpoint of 75 deg F, then you should be supplying air at a 20 deg delta T, so in this case your 'Zone Cooling Design Supply Air Temperature' would be 55 deg F = 12.8 deg C
Generally engineers will select systems using this 55 deg F / 12.8 deg C supply air temperature for cooling
Now for the heating conditions, with a VAV with HW reheat system, the air temperature leaving the AHU in heating mode needs to be less than the cooling design setpoint, so the 'SYSTEM Heating Design Supply Air Temp' here would be 12 or so, and the 'ZONE Heating Design Supply Air Temp' would be 90 deg F or 32.2 deg C for a zone heated to 70 deg F
Hope this helps,