| Data collection rule (DCR) | Azure Monitor uses the [Data collection rule (DCR)](../essentials/data-collection-rule-overview.md) to understand the structure of the incoming data and what to do with it. If the structure of the table and the incoming data don't match, the DCR can include a [transformation](../essentials/data-collection-transformations.md) to convert the source data to match the target table. You can also use the transformation to filter source data and perform any other calculations or conversions.<br><br>If you create a custom table using the Azure portal, the DCR and the transformation are created for you based on sample data that you provide. If you use an existing table or create a custom table using another method, then you must manually create the DCR using details in the following section.<br><br>Once your DCR is created, you must grant access to it for the application that you created in the first step. From the **Monitor** menu in the Azure portal, select **Data Collection rules** and then the DCR that you created. Select **Access Control (IAM)** for the DCR and then select **Add role assignment** to add the **Monitoring Metrics Publisher** role. |
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