Ryan…
When you use the polygon tool in photoshop to create a vector shape, typically Photoshop creates a layer (the fill) with a vector mask. You can edit the vector mask shape by going to the Paths pallette and using the pen tools. As you alter the shape, it will reveal or hide the fill accordingly.
The fill is what’s being masked.
Hope that helped.
–Scott
Yes, that would make a difference.
When making a selection with the Lasso tools, you just end up with a selection. It’s now up to you what you want to do with it. You could just create a new layer and fill the selection with a color or gradient. Or you could create or duplicate a layer and use the selection as a layer mask. In the latter, you have the same situation as in my last post, however, the mask is not automatically a path that can be manipulated by the pen tools. But, it could be converted into a path, manipulated, turned back into a selection and reapplied as the mask. Or you could just paint with black and white in the mask to hide or reveal the fill.
–Scott