By scrolling through the outline you can find brief discussions of this short portion of Kant's text. This distinction between empirical and rational knowledge rests on a difference in sources of evidence used to support the two different kinds of knowledge.
Material knowledge is of a specific subject matter, either nature or freedom.
Note that there is no formal, empirical knowledge. It articulates the rules which govern all thought about anything. It develops general standards or criteria of truth and validity. Such material, rational knowledge is metaphysics, of which there are two branches, the metaphysics of nature and of morals. In addition to the metaphysics of nature, an apriori physics or rational knowledge of nature, Kant wants to wants to develop an apriori or pure moral philosophy , a Metaphysics of Morals.