ImageWorkshop could be used as a document.

1 - What is a document

A document is the group that contains all your subgroups and sublayers. We just call document the group which will include all our layers, and with which you will generate the resulting image.
It could be preferable to call explicity the variable of your document so, because other developpers on your project will directly understand that this variable is the main group, but this is not mandatory !

2 - How to use

A document is a group, this is also a layer. It works like a layer.

Now that you're familiar with the different names of a layer, we will see how to add a layer or a group in the stack of another group.