What is the lighter area of the today pane?

I noticed there’s a lighter area in the today pane on iOS. Some widgets will only live in that area, and some others just in the area outside. **It seems there are two kinds of widgets.** Why is this so? In the images below the first widget (weather) is of one type and can only be placed outside of the lighter (on dark mode darker) area. The other widget can only be placed in it. After haptic touching the today pane, one can edit the two kinds of widgets in different ways: the first kind by clicking the + and the second kind by clicking Anpassa (edit). The lighter area is for one kind of widgets, it seems.