IImfChannelList.h:

    // Support for image layers:
    //
    // In an image file with many channels it is sometimes useful to
    // group the channels into "layers", that is, into sets of channels
    // that logically belong together.  Grouping channels into layers
    // is done using a naming convention:  channel C in layer L is
    // called "L.C".
    //
    // For example, a computer graphic image may contain separate
    // R, G and B channels for light that originated at each of
    // several different virtual light sources.  The channels in
    // this image might be called "light1.R", "light1.G", "light1.B",
    // "light2.R", "light2.G", "light2.B", etc.
    //
    // Note that this naming convention allows layers to be nested;
    // for example, "light1.specular.R" identifies the "R" channel
    // in the "specular" sub-layer of layer "light1".
    //
    // Channel names that don't contain a "." or that contain a
    // "." only at the beginning or at the end are not considered
    // to be part of any layer.