# Guidelines and requirements
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Each tag consists of a required tag key and an optional tag value, both of which you define. A tag key is a general label that acts as a category for more specific tag values. A tag value acts as a descriptor for a tag key.

For example, if you have two versions of an Amazon Personalize dataset group (one for internal testing and another for production), you might assign an `Environment` tag key to both projects. The tag value of the `Environment` tag might be `Test` for one version of the dataset group and `Production` for the other version.

The following restrictions apply to tags:
+ Maximum number of tags per resource – 50
+ Maximum key length – 128 Unicode characters in UTF-8
+ Maximum value length – 256 Unicode characters in UTF-8
+ Tag keys and values can contain the following characters: A-Z, a-z, 0-9, space, and \$1 . : / = \$1 @ – (hyphen). This is the standard set of characters available across AWS services that support tags. Some services support additional symbols.
+ Tag keys and tag values are case sensitive.
+ For each associated resource, each tag key must be unique and it can have only one tag value.
+ Your tag keys and tag values can't start with `aws:`. AWS services apply tags that start with `aws:`, and those tags can't be modified. They don't count towards tag limits.
+ You can't update or delete a resource based only on its tags. You must also specify the Amazon Resource Name (ARN) or resource ID, depending on the operation that you use.

## Additional information
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For more information about tagging, see the following resources.
+ [AWS Tagging Principles](https://docs.aws.amazon.com/general/latest/gr/aws_tagging.html) in the *AWS General Reference*
+ [AWS Tagging Strategies](https://d1.awsstatic.com/whitepapers/aws-tagging-best-practices.pdf) (downloadable PDF)
+ [AWS Access Control](https://docs.aws.amazon.com/IAM/latest/UserGuide/access_tags.html) in the *AWS IAM User Guide*
+ [AWS Tagging Policies](https://docs.aws.amazon.com/organizations/latest/userguide/orgs_introduction.html) in the *AWS Organizations User Guide*