# Code lists

A code list is a controlled set of values for an attribute. This section identifies the code lists used within OS Open Rivers and describes their values:

* [LinkDirectionValue](https://docs.os.uk/os-downloads/products/water-portfolio/os-open-rivers/os-open-rivers-technical-specification/code-lists/linkdirectionvalue)
* [WatercourseFormValue](https://docs.os.uk/os-downloads/products/water-portfolio/os-open-rivers/os-open-rivers-technical-specification/code-lists/watercourseformvalue)
* [HydroNodeCategoryValue](https://docs.os.uk/os-downloads/products/water-portfolio/os-open-rivers/os-open-rivers-technical-specification/code-lists/hydronodecategoryvalue)