SDL_!QuitEvent? is a member of the SDL_Event union and is used whan an event of type SDL_QUIT is reported.
As can be seen, the SDL_!QuitEvent? structure serves no useful purpose. The event itself, on the other hand, is very important. If you filter out or ignore a quit event then it is impossible for the user to close the window. On the other hand, if you do accept a quit event then the application window will be closed, and screen updates will still report success event though the application will no longer be visible.