NiFi allows you to specify multiple variable registry files in the nifi.properties file. There is only the one property:
This is a comma-separated list of file location paths for one or more custom property files. These files are only read by NiFi on startup, so any changes or additional files added to this property will not take affect until you restart NiFi.
The variable registry file was added to NiFi before it became possible to set NiFi Variables directly within the NiFi UI on process groups. The advantage to the newer variables that can be set on process groups is that additional variables and changes to existing variables can be made without needing to restart NiFi. From the NiFi Ui you can see a list of all components that are currently using a set variable. Changing a variable via the UI also will handle stopping and restarting any component already referencing that variable.
Bonus: Keep an eye out for "NiFi Parameters" coming soon. Unlike variables, parameters can be used in any NiFi component property field. Variables are only support in component properties that support NiFi Expression Language (EL). Parameters will also support sensitive properties (meaning they are able to store passwords in a secure encrypted format.