# id (hru-data.hru)

The ID of the HRU is a primary key referenced by [hru](https://swatplus.gitbook.io/io-docs/introduction-1/connectivity/hru.con/hru) in [**hru.con**](https://swatplus.gitbook.io/io-docs/introduction-1/connectivity/hru.con). All IDs in the **hru-data.hru** file must be unique.