Canvas
From Synfig Studio :: Documentation
A canvas is simply an ordered list of layers. When you Group a set of layers, you are making a new Canvas, putting the grouped layers into that new canvas, removing them from the Canvas they were previously in, and replacing them with a new Group Layer. The new Group Layer has a parameter called "Canvas" which has the new Group Layer name as its value.
Canvases can be exported (the name must not contains space or :#@$^&()* characters) , which adds them to the Canvas Browser Panel, from where they can be edited separately in their own workarea.
Sub-canvases can in turn be exported from these new workareas, building up a tree of canvases.