The Hiera configuration file is called hiera.yaml. It configures the hierarchy for a given layer of data.
Location of hiera.yaml files:
There are several hiera.yaml files in a typical deployment. 
Hiera uses three layers of configuration, and the module and environment layers typically have multiple instances.
The configuration file locations for each layer:
Layer                                     Location
Global                                   $confdir/hiera.yaml
Environment                        <ENVIRONMENT>/hiera.yaml
Module                                <MODULE>/hiera.yaml