At the bottom of this article there is some explanation of the Tornado requirements. https://www.mecaenterprises.com/asce-7-22-wind-loads/
The MecaWind software helps you through the tornado process. For example, you only have to consider Tornado for Category III and IV structures, and so if you have Category I or II selected then you won't even see the tornado option. When the tornado option is selected, you can click the button to go to the ASCE Hazzard site to determine what wind speed to use for Tornado for a given location.
After you enter the Tornado wind speed it goes through the checks in ASCE 7 to see if Tornado is required to be checked. If it is required then it performs those checks, and if it isn't required then it will indicate why tornado doesn't need to be checked.