In addition to the x-, y-, and z-coordinates that define space in a Cartesian coordinate system, Direct3D uses texture coordinates. These coordinates (u and v) define an "up" direction for the texture, typically along the y-axis (u) and an orientation along the plane of the texture, typically along the z-axis (v). As with all normalized vectors, the origins of these vectors are at [0,0,0].
For more information about texture coordinates, see Textures.