After a texture has been mapped to a surface, the texture elements (texels) of the texture rarely correspond to individual pixels in the final image. A pixel in the final image can correspond to a large collection of texels or to a small piece of a single texel. You can use texture filtering to specify how to interpolate texel values to pixels.
You can use the IDirect3DRMDevice::SetTextureQuality method and the D3DRMTEXTUREQUALITY enumerated type to specify the texture filtering mode for your application.