A photographic mask is a secondary image made from the original image and combined, in register with another element, [negative or positive] in order to alter the elements densities on printing. Masks were widely used to make major changes in contrast, and to cancel out the unwanted absorptions of poor subtractive dyes, in early paper print processes like the Dye Transfer Process.
In motion picture laboratories masks are hardly ever used for these purposes, as few printers can handle a "sandwich" of film without difficulty and registration is difficult to maintain. Masks are used for special effect production such as the Travelling Matte system. Masks used for this purpose are called Mattes.
A negative mask combined with a negative will increase the density range and therefore the contrast of the combined image. Conversely a positive mask combined with a negative will reduce the density range and therefore the contrast. There are several references in the reference section of this publication, but in archive work the need for mask making is limited to the correction contrast caused by faded dyes in old colour films, but is very rarely undertaken.