There are actually 3 sets of filters. We called them Status filters and they were:

Status A : Positive films
Status M: Negative films
Status D: Reflection print materials

Ok, now why without technical details....

The reversal films are designed to be viewed by the eye as a reflected image from a screen through projection. The Status A filters are designed to look at reversal films in such a way as to pretty much duplicate the response as you would see it.

The color negative film is designed to be printed to form a reversal image on film or paper. The filters therefore mimic the response of the print material to the dyes.

The color papers cause some degree of "distortion" of the dye image due to the reflective back and the internal reflections. Therefore, the filter set is designed to "see" what the human would see due to the distortion.

You get quite different curves from each filter set, but if you get used to using a "wrong" set, you can make do quite easily, but there is a limit if you are doing ultra critical work. In some cases, you can see a neutral but the densitometer will not read it as a neutral - or vice versa.

PE