Although cacao powder is derived from cacao nibs, grinding cacao nibs will not immediately yield this powder. The composition of the nibs is approximately half fatin the form we call cocoa butterso grinding nibs will instead yield a liquor. (Incidentally, is the base for making chocolate bars and treats.) To obtain powder this liquor is further processed, usually by using industrial devices such as a hydraulic presses in order to separate out the fat.
The producer can choose how much fat to leave in their powder. High fat powder contains around 20-24% cocoa butter, and is usually more expensively priced than the 10% cocoa fat counterparts (FDA requires a 10% minimum for powder). Higher fat will yield a richer chocolate flavor and more moist baked goods.
It will also be less soluble in cold liquids due to its fat content. Used in baked goods to create chocolate flavor and chocolate brown color. When used in baking to supplement other chocolates in the recipe it will further enhance the chocolate flavor giving it more depth and complexity.