Puzzle Games Tutorial

Hi, I am going through the Puzzle Game tutorial in TDN and had a question about the script block under the "Building Blocks" section particularly regarding the ShapeTemplate::onAdd() function. I understand what the function is doing, but I am not sure where the "shapeGroup" variable came from or what type of variable it is.

Here is a snippet of the script I am having difficulty parsing.

if(!isObject(shapeGroup))
new SimSet(shapeGroup);

...

shapeGroup.add(%this);

In the first line of the script it checks to see if it exists and if it doesn't it creates a new SimSet which I am understanding is a collection of SimObjects. So my first question is does "shapeGroup" exist elsewhere? Also, what type of variable is it since there isn't either '%' or '$' preceeding it? Another question that popped up is why is it being passed to the SimSet constructor as a parameter? I looked up the SimSet reference and I didn't see anything regarding a parameterized constructor for SimSet.

Thanks in advance for your help.