Rem-Jet is, first and foremost but not exclusively, a type of anti-halation coating. MP film uses Rem-Jet for anti-halation because it offers other benefits for MP film - it has an anti-static and a lubricating function which is beneficial for MP film which is intended to be always 'on the move' in the camera (unlike still film). Differt films for different applications use different methods for anit-halation. BW neg has a neutral grey base. Some BW reversal films (Foma in particular comes to mind) uses an opaque silver layer under the emulsion which is bleached off in the reversal process. Colour neg still film and colour reversal MP and still films use a dye layer wich is under the emulsion and which is disolved out in processing.
Here is some info from a kodak page on film structure:
Three antihalation methods are commonly used:
• Remjet, a removable jet black layer, is the coating of carbon black particles in a water-soluble binder on the bottom of the film. It has four purposes: antihalation, antistatic, lubrication, and scratch protection. The remjet carbon layer is also conductive and prevents the build-up and discharge of static charges that can fog film. This is especially important in low relative humidity environments. Remjet also has lubricating properties. Like the supercoat on top of the emulsion, remjet resists scratching on the base side and helps transport the film through cameras, scanners, and printers.
Because remjet is black, it must be removed before the image can be seen. Remjet is removed during the first stage of processing, before the developer.
• Antihalation undercoating, a silver or dyed gelatin layer directly beneath the emulsion, is used on some thin emulsion films. Any color in this layer is removed during processing. This type of layer is particularly eEective in preventing halation for high resolution emulsions. An antistatic and/or anticurl layer may be coated on the back of the film base when this type of antihalation layer is used.
• Dyed film base serves to reduce halation and prevent light piping. Film base, especially polyester, can transmit or pipe light that strikes the edge of the film and result in fog. A neutral-density dye is incorporated in some film bases to mitigate this eEect. Dye density may vary from a barely detectable level to approximately 0.2. Higher levels are primarily used for halation protection in black-and-white negative films on cellulose bases. Unlike fog, the gray dye doesn’t reduce the density range of an image; it adds the same density to all areas just as a neutral-density filter would. It has, therefore, a negligible eEect on picture quality.http://motion.kodak.com/motion/uploaded ... -image.pdf