The hierarchy
The hiera.yaml file configures a hierarchy: an ordered list of data
sources.
Hiera searches these data sources in the order they are written. Higher-priority sources override lower-priority ones. Most hierarchy levels use variables to locate a data source, so that different nodes get different data.
This is the core concept of Hiera: a defaults-with-overrides pattern for data lookup, using a node-specific list of data sources.
Related information