Imagine a t-shirt product I'd like to buy for my friend. I see the style node (the t-shirt) which is a product class with two attributes:
My measurements - which will show me a list of all my friends and their body measurements. This list will be populated by a Views query (views.module).
Fabric width - which will show me the different thickness of the fabric. This list needs to change according to the user profile's working units (profile.module)
Here's how you define it:
1. Create attributes with one option, which is called dummy.
2. Create a class and assign it the attributes (snap1).
3. Enable the attached module, and check your add to cart (snap2).
Note that the change in the 'add to cart' block will be visible only in new content types, i.e. it will not appear in older nodes
btw, we've added it under code snippets since it's not generic enough to be a module. Just a workaround.