In CMYK printing, registration black refers to 100% coverage in each of the four process colors: cyan (C), magenta (M), yellow (Y), and black

Black

Black is the color of objects that do not emit or reflect light in any part of the visible spectrum; they absorb all such frequencies of light...

 (K). More generally, if a different mixture of colors is being used, registration black marks in each of the colorants (inks) used.

It is therefore not correct to simply define registration color as 100% of C,M,Y,K, since it will rather be 100% of whichever colors are used in reproduction.

It is a special purpose color, and as such is not generally used to print black text or grayscale

Grayscale

In photography and computing, a grayscale or greyscale digital image is an image in which the value of each pixel is a single sample, that is, it carries only intensity information...

 graphics.

Instead, registration black is used for printing crop marks, or "registration marks". When proofs for each color are generated on separate pieces of film, use of registration black makes crop marks visible on all channels, providing a useful reference for alignment. A thin line printed in registration black can also be used to check whether the printing plates are lined up.

PostScript

The PostScript

PostScript

PostScript is a dynamically typed concatenative programming language created by John Warnock and Charles Geschke in 1982. It is best known for its use as a page description language in the electronic and desktop publishing areas. Adobe PostScript 3 is also the worldwide printing and imaging...

 printer description languages supports registration black, starting with PostScript language level 2. This is done by referring to a spot color

Spot color

In offset printing, a spot color is any color generated by an ink that is printed using a single run.The widely spread offset-printing process is composed of four spot colors: Cyan, Magenta, Yellow, and Key commonly referred to as CMYK...

 with the special name All. This never generates a spot plate. Instead it marks all of the plates that are there. The All color space can be used with a tint value between 0.0 (no mark) to 1.0 (full intensity). Generally, only 1.0 would be used.

The name "All" might not be used in the user interface of a design program, especially outside English language speaking areas. However, the spot color must have the exact name "All". As a side effect, it is impossible in PostScript to create a normal spot plate with this name.

PDF

The Portable document format

Portable Document Format

Portable Document Format is an open standard for document exchange. This file format, created by Adobe Systems in 1993, is used for representing documents in a manner independent of application software, hardware, and operating systems....

 (PDF) also includes a spot color called All, with the same restrictions, starting with PDF 1.2. Note that a PDF spot color must also include a "tint transform" which translates spot values into a different color space

Color space

A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...

for viewing on screen, or printing to printers without spot color support. There is no special rule for the name "All", so PDF creators must include a tint transform that converts to black in some color space, in order to maintain the same appearance as the final printed piece.