Tag URI is a universal unique identifier

Unique identifier

With reference to a given set of objects, a unique identifier is any identifier which is guaranteed to be unique among all identifiers used for those objects and for a specific purpose...

 protocol commonly used in various internet-related technologies and associated with data storage and transfer systems such as YAML

YAML

YAML is a human-readable data serialization format that takes concepts from programming languages such as C, Perl, and Python, and ideas from XML and the data format of electronic mail . YAML was first proposed by Clark Evans in 2001, who designed it together with Ingy döt Net and Oren Ben-Kiki...

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According to RFC 4151, a tag is a type of Uniform Resource Identifier

Uniform Resource Identifier

In computing, a uniform resource identifier is a string of characters used to identify a name or a resource on the Internet. Such identification enables interaction with representations of the resource over a network using specific protocols...

 (URI) designed to meet the following requirements:

1. Identifiers are likely to be unique across space and time, and come from a practically inexhaustible supply.
2. Identifiers are relatively convenient for humans to mint (create), read, type, remember etc.
3. No central registration is necessary, at least for holders of domain names or email addresses; and there is negligible cost to mint each new identifier.
4. The identifiers are independent of any particular resolution scheme.

Examples

The following is an example tag that could be "minted" by a user to indicate authorship of a resource on the World Wide Web

World Wide Web

The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...

. The semantics of Tag URI do not require the user to currently have ownership of the Hostname

Hostname

A hostname is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication such as the World Wide Web, e-mail or Usenet...

upon which the tag is based, due to the date qualifier:

tag:user@example.com,2007-11-02:Tag_URI

The following example is a similar tag minted to indicate the name of the pet gerbil of the same user. Note that the semantics of Tag URI do not require the tagged entity to be accessible electronically:

tag:user@example.com,2006:Fluffy

This demonstrates that the Tag URI scheme can be used to tag any entity, object or concept.