Indirect key-based addressing
DITA keys provide an alternative to direct addressing. The key reference mechanism provides a layer of indirection so that resources (for example, URIs, metadata, or variable text strings) can be defined at the DITA map level instead of locally in each topic.
For information about using keys to define and reference controlled values, see Subject scheme maps and their usage.
The material in this section of the DITA specification is exceptionally complex; it is targeted at implementers who build processors and other rendering applications.