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.
Sub-canvases can in turn be exported from these new workareas, building up a tree of canvases.
Creating a new Canvas
Canvases can be created by right-clicking on the Canvas parameter of a Group layer and choosing Export Value.
In the dialogue that appears enter a name for your new canvas.
The canvas will now be created in the Canvas Browser to the right of the screen. Double-clicking on the canvas name will open the canvas, which contain all of the layers that are part of the original Group