This window opens if you go to the top menu bar and select World → Environment Editor → Environment Settings. Here you may control how you see the sky and water in a region. You can opt to use region settings, or your own.
Always use parcel/region settings:
If this is enabled, the Windlight settings for the region or parcel you are in, will always be applied, overriding whatever personal settings you might have. Disable this if you prefer to use your own settings always. The same as Preferences
→ Windlight → Automatically change environment to use region/parcel settings.
Use region settings: Enable this if you want region settings to be used when you arrive in a region. You can also use this to switch from forced time of day to the current region settings.
Customize my environment: If you prefer to have custom Windlight settings, enable this. Then the items below will become available.