# Using cost allocation EFS file system tags
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To track the storage cost or other criteria for individual projects or groups of projects, label your EFS file systems using cost allocation tags. A *cost allocation tag* is a key-value pair that you associate with a file system. After you activate cost allocation tags, Amazon uses the tags to organize your resource costs on your cost allocation report.

The *cost allocation report* lists the Amazon usage for your account by product category and linked account user. The report contains the same line items as the detailed billing report (see [Understanding billing and usage reports for Amazon EFS](billing-usage-reports-understand.md)) and additional columns for your tag keys.

Amazon provides two types of cost allocation tags, *Amazon-generated tags* and *user-defined tags*. Amazon defines, creates, and applies the Amazon-generated `aws:createdBy` tag key for you after an Amazon EFS CreateFileSystem event. You define, create, and apply *user-defined* tags to your file systems.

You must activate both types of tags separately in the Billing and Cost Management console before they can appear in your billing reports. For more information about Amazon-generated tags, see [Organizing and tracking costs using Amazon cost allocation tags](https://docs.amazonaws.cn/awsaccountbilling/latest/aboutv2/cost-alloc-tags.html) in the *Amazon Billing User Guide*.
+ For more information about creating user-defined tags for EFS resources, see [Tagging EFS resources](manage-fs-tags.md).
+ For more information about activating user-defined tags in the Billing and Cost Management console, see [Activating user-defined cost allocation tags](https://docs.amazonaws.cn/awsaccountbilling/latest/aboutv2/activating-tags.html) in the *Amazon Billing User Guide*.
+ For more information about activating Amazon-generated tags in the Billing and Cost Management console, see [Activating Amazon-generated cost allocation tags](https://docs.amazonaws.cn/awsaccountbilling/latest/aboutv2/activate-built-in-tags.html) in the *Amazon Billing User Guide*.

## User-defined cost allocation tags
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A user-defined cost allocation tag has the following components:
+ The tag key. The tag key is the name of the tag. For example, in the tag project/Trinity, project is the key. The tag key is a case-sensitive string that can contain 1 to 128 Unicode characters. 
+ The tag value. The tag value is a required string. For example, in the tag project/Trinity, Trinity is the value. The tag value is a case-sensitive string that can contain from 0 to 256 Unicode characters.

For more information about user-defined tags, see [Using user-defined cost allocation tags](https://docs.amazonaws.cn/awsaccountbilling/latest/aboutv2/custom-tags.html) in the *Amazon Billing User Guide*.