Regarding all Adobe software... primarily Illustrator as well as InDesign....
Registration swatch is only to be used in two instances:
- When you are manually drawing printer marks, i.e. trim, bleed,
targets, die lines.
- When you are working on a CMYK document and want black on a mask,
such as an Opacity mask in Illustrator.
These are the only two cases to use the
Registration is a color created by using 100% Cyan, 100% Magenta, 100% Yellow and 100% Black - totaling 400% of ink. Almost every commercial print provider will not accept a file where that 400% color build is used within the artwork. It's impossible to maintain on a press because it is too much ink and any stock (or paper) will not absorb that much ink and prints will run, drip, and smear.
Never use the
Registration swatch for general artwork coloring. Not ever.
Regarding it's general use in InDesign and masks....
In addition to creating your own printer marks or page info, the Registration swatch can be very handy in creating masks.
Often a mask consisting of only K will not completely mask a CMYK object. So, using Registration for the mask "black" tends to be more complete.
It's not very easy to see in InDesign specifically, but "feather" effects utilize the "Registration" Swatch:
You can't change that swatch, because it's designed to hide all colors via the registration swatch. The other feather options don't even show you the color because it's not necessary. However, they also use "Registration" to mask everything.
Manually, use of the Registration swatch is more easily seen in Illustrator with Opacity Masks. Same theory, just more automated in InDesign.