Foe short, if any objects in your model have aurosize value, you need to perfoem sizing calculation.
The "Ideal Zonal System" and "Ideal Loads Sizing Simulations" sizing calculations are the same thing. They only work in zone sizing period. The results from the zone sizing period will be used to determine the sizes 'authorize' values in some of HVAC systems in the objects. For zone sizing, it determines the macimum supply air flow rate based on the design day conditions. For system sizing. sizing, it gets the DX coil maximum capacitits and so on. For plant sizing, it finds the chiller and boiler capacities.

The "run simulation for sizing periods" is similar to "Run Simulation for Weather file run periods". The "Run simulation for sizing periods" use the "SizingPeriod:" There are lots of objects as the weather data, while the "Run Simulation for Weather file run periods" use "RunPeriod:" objects as the weather data. Both results are outputs to the *.csv and other output files.with autosize capabilities.

Foe short, if any objects in your model have aurosize value, you need to perfoem sizing calculation.
The sizing calculations are used to determine the 'authorize' values in some of the objects. For zone sizing, it determines the macimum supply air flow rate based on the design day conditions. For system sizing, it gets the DX coil maximum capacitits and so on. For plant sizing, it finds the chiller and boiler capacities.