IWAD selection box
The IWAD selection box is a special widget shown if more than one IWAD are detected when ZDoom is started and before data is loaded. This dialog serves only to select which IWAD is used as the base game: it is not a full-fledged frontend. Think of it purely as a game selector, not a mod selector. For loading mods, please refer to this article.
The dialog remembers which was the last choice made and will preselect it. If the "Don't ask me this again" box is checked, this choice will be kept for each run of ZDoom, until it is enabled back. This can be done from the menu, by editing the configuration file, by changing the variable through the console or menu, or by holding the "Shift" key while starting ZDoom.