A texture coordinate determines where to sample a texture. When a texel does not map exactly to one pixel, filtering is necessary to magnify (enlarge) or minify (shrink) the texture.
Not all valid filtering modes for a device apply to volume maps. In general, Point and Linear magnification filters are supported for volume maps. Devices that support anisotropic filtering for 2D maps do not necessarily support it for volume maps. However, applications that enable anisotropic filtering, even if they do not support it, receive the best available filtering (probably linear).