In the modern color-managed workflow, an RGB image has an associated profile so each RGB number combination can be converted to a defined Lab color. This is fairly straight-forward and repeatable. When creating a CMYK combination to represent that color on output, things get considerably more complicated.
Lets start by showing how color is created using CMYK. If you apply yellow ink to paper, your color range starts at paper white and then becomes more yellow and saturated with the more ink you apply. But once you get to 100% yellow there’s nothing more you can do without adding other inks. If you are looking for a medium/dark yellow you now have a whole host of choices to get it.

• First, you can add cyan and magenta ink. They are both required in order to offset each ink’s tendency to move the color toward green or red. The addition of cyan and magenta does darken the yellow but they are also, together, blue – which is anti-yellow. So this addition of blue desaturates the yellow ink quickly, limiting the range of dark yellow colors available.
• A second choice is to add black ink. As black is added, the yellow darkens but is not desaturated nearly as quickly. This can result in a greater gamut of dark yellows.
  

When CMYK colors are created in normal workflows, either or both of these techniques are used. In fact, for a single original RGB color, many different combinations of CMYK can be used to (theoretically) create the same color on a CMYK printer or press.
      So how do we decide which one of the above to use?
      How much black should be used instead of CMY?
The answer, as you should already have guessed, is the classic color management answer: “It depends”.
Lets cover a bit more technique and terminology first….

Gray Component Replacement (GCR)

Cyan, magenta and yellow inks used for offset printing are not pure enough colorants to be mixable in equal amounts for gray. The ‘SWOP’ standard for instance, expects 50% cyan and 40% each of magenta and yellow inks to produce a neutral or near neutral gray. So let’s say we had a CMY color of 60, 50, 40. In theory, if we removed the components of the color that produced gray (50,40,40) we would then be left with (10,10,0). If we then add enough black ink to bring us down to the same darkness (about 54% K), we have the CMYK combination of 10,10,0,54 that should appear very close to the original color yet is composed of VERY different amounts of ink! The total ink used drops from 150% to 74% and
changes from the more problematic, in terms of density fluctuations on press and/or
metamerism, colored inks to black ink. This kind of color replacement is called Gray Component Replacement (GCR).

Under Color Removal (UCR)

If the color range affected by this replacement is limited to dark, near-neutral colors, then it is called Under Color Removal (UCR).

GCR, on the other hand, can be applied to neutral and non-neutral colors that are either light or dark. This is basis for ink saving and press stability.
Restricting black replacement to neutrals is what UCR is all about. GCR came along later in the history of color management and extended this technique beyond the neutrals and into the colors. If done correctly, GCR can be very effective and improve image quality. Early GCR didn’t always work as expected and many fell back to UCR for safety. GCR is now at the point where it is reliable and effective, and UCR is falling out of use. As the function of UCR is a special case of GCR (dark neutrals only), expect the term UCR to fall out of use and we will then refer to all black replacement as GCR.

Black start / max / width

In ICC profiles, the black generation method and amount is chosen at the time the profile is calculated and “baked” into the profile. For flexibility, we suggest calculating several profiles from the same measurements; each with different black generation settings.

Black Start is the amount of cyan ink where black starts replacing other inks. For instance, a black start of 10 means that when cyan ink is less than 10%,
CMY will be the only inks used to create colors, but for any colors where cyan is greater than 10%, K will replace some component of the color.

Black Max is how high you want the K level to get in the resulting color. If you find your shadows are plugging, reducing Max K can help.

Finally, black width is how far “out” into the saturated colors you want black to be substituted. A low black width will limit K substitution to near-neutral colors (similar to UCR). A high black width will allow substitution much farther out into the saturated colors. If you find your saturated colors look muddy, try lowering your black width.