> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/introduction-1/routing-units/untitled-1/id-rout_unit.def.md).

# id (rout\_unit.def)

The ID of the Routing Unit is a primary key referenced by [define](/io-docs/introduction-1/routing-units/untitled/define.md) in [**rout\_unit.rtu**](/io-docs/introduction-1/routing-units/untitled.md). All IDs in the **rout\_unit.def** file must be unique.