Prompted by the current discussion about Kant’s concept of causality of freedom http://philosophy.stackexchange.com/questions/28746/can-free-will-exist-in-a-causal-material-world I would like to understand the scope and the difference of the two Kantian concepts

 - Freedom as a *transcendental* idea
 - Freedom in its *practical* sense.

See Critique of Pure Reason (CPR), B561ff. Currently, I understand these two concepts as follows:

An idea is a *transcendental* idea if it refers to our way to create experience – but not to experience itself. In particular, a transcendental idea lives on the meta-level, it belongs to epistemology.

A concept is *practical* if it has a normative component, i.e. when the concept considers what should be done. In particular, a practical concepts belongs to ethics. 

If you can confirm my understanding of these two concepts, I would like to know – according to Kant

- Why do we need freedom as a transcendental idea?
- Why does freedom in its practical sense describe our experience?

**Note**. I would wellcome a direct answer, not an invitation to a tour de force through the secondary literature :-)