Clicking on the [...] button brings up the File Browser for selection of the environment. Choose the Library category for a selection of pre-configured environments. If you have created your own and stored it inside your park's folder, you can select it from the Park Base category.
Note that you have to save, close and re-open the park in order for the changes to take effect.
Dynamic Sky tab allows you to override the dynamic sky configuration in the settings menu. This allows you to specify the time of day and date instead of leaving it to the dynamic configuration set by others who are visiting your park.
Weather tab enables controls to override the default weather. This is useful when you want to ensure your coaster is always ridden while it is snowing or on a dark and stormy night.