# id (rout\_unit.rtu)

The ID of the Routing Unit is a primary key referenced by [rtu](https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/introduction/connectivity/hru.con/hru) in [**rout\_unit.con**](https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/introduction/connectivity/hru.con). All IDs in the **rout\_unit.rtu** file must be unique.
