What I found out so far: there is a little bit of (hidden as “legacy”) documentation:
http://docs.wpecommerce.org/checkout/
*Add a new form set*
This creates a new form set for you to use – This is the best way to create a custom checkout (if you don’t want to use the default).
It can also be used if some product categories require additional information to be collected at checkout as you can apply a custom form set to a category, the fields in this form set will then be applied to every product in that category at checkout.
What I am looking for is the functionality described in the first paragraph…
