Here's what each layer does:
Pasteboard background layer
The Pasteboard background layer serves as a desktop-like surface for you to work on.
On this layer, it is possible to change colors and configure images, but no interaction is possible.
When you create a new page, each element on this layer is normally applied to that page. There is normally only one element on this layer.
This layer takes up the full width of the browser.
Page background layer
This layer is like a piece of paper on the desktop. Like the pasteboard background layer, each element on this layer is normally added to all new pages you create. This layer also does not permit interactions with elements on the layer (you cannot click or select them). However, the elements placed on this layer are displayed in the same position in the browser as they appear in Web Designer.
This layer is where you should place all the interactive elements of your website. Elements added to this layer are not automatically applied to new pages. Each element on this layer becomes visible as soon as the page loads (unless a special behavior has been defined for the element).
Elements on this layer only become visible when the user places their mouse cursor over them and only if a link has been placed at the same position on the MouseOff Layer.