A CC3 sheet is a container that holds drawing items. It controls the order in which things are drawn; that is everything on one sheet will be drawn, then everything on the next sheet, etc.
A CC3 Layer, on the other hand, doesn't control the order of drawing. In CC3 a layer is simply an attribute of a drawing item that gives control over some other attributes such as visibility.
If you're familiar with drawing packages like Photoshop then you will likely be very familiar with CCx's sheets. What CC3 refers to as "layers" will likely unlike anything you have dealt with in other packages. When Profantasy was asked some years back why sheets are layers and layers are unfamiliar the answer was along the lines of "we were calling them layers long before these upstart drawing packages were whelped and that makes our terminology correct!"
CC3 is powerful, but the terminology and operating techniques require unlearning if you've used a typical modern GUI software suite before.