Sheets vs. Layers in CC3:

In CC3, Sheets function like how you would expect layers to act in other programs - Sheets are drawn on top of each other (which is why an entity on one Sheet can cover or obscure the view of an entity on a lower Sheet) in a "stack" and establish the drawing order. CC3's Sheet Effects (shadows, glows, blurs, transparencies etc.) are applied to Sheets as well.

Think of Layers in CC3 as categories instead. You can group like entities on a Layer, even if they reside on different Sheets.