# COU supplies

Change-Only Updates (COUs) are only available for CSV data supplies of the OS NGD. The benefit of taking COU supplies is that you only receive the features which have changed since your last supply, reducing the overall size of the files you receive.

There are some restrictions on when you can choose to take COUs; these restrictions are dependent on the selections you make when ordering OS NGD data. Please see the ['Data ordering and currency' page](https://docs.os.uk/osngd/getting-started/os-ngd-fundamentals/os-ngd-data-ordering-and-currency) for more information.

Upon OS NGD launch, if you choose to receive a COU supply for your CSV data, this can be provided on either a daily or monthly frequency. In both cases, your COU file will contain any changes which have been made to the underlying data within the selected time frequency. If no changes have been made to a selected feature type of your choice, then you will receive a blank COU file.

The key attribution required to use and implement a COU is detailed below:

* **Unique Identifier**: The unique identifier for the feature type you are using is how you will ensure you are updating the correct records in your data holdings. In most instances the unique identifier will be the OSID, but this is not always true, therefore please check the appropriate feature type pages in our documentation.
* **Version Date**: This is the date when the feature was last updated in OS product databases.
* **Version Available From Date**: This is the date when the feature became the latest version and was published to customers.
* **Version Available To Date**: This is the date when the feature was superseded by an update or ceased to exist.
* **Change Type**: This is a code list which gives more information about the type of change which has occurred.

{% hint style="info" %}
Please note, a single feature may be updated multiple times in a single COU file. This would happen where a single feature has changes committed in our product databases more than once within your selected COU frequency. Instead of suppressing all changes other than the last edit, your COU will contain all edits we have made to that feature. This is likely to occur more commonly if you choose to take a monthly supply, compared to daily, but it could also occur in a daily COU.
{% endhint %}

{% hint style="info" %}
You can use the [OS Downloads API](https://osdatahub.os.uk/docs/downloads/overview) to manage your daily / monthly COU supply of OS NGD data.
{% endhint %}

{% hint style="info" %}
Please be aware that a feature can move between OS NGD feature types and even between OS NGD themes when OS makes data improvements or, in some instances, when a feature undergoes a change. For example, Pre-Build Address features will naturally change to Built Address features over time.
{% endhint %}


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