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tag URI scheme
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<p>The tag URI scheme is a <a href="/facts/Uniform_resource_identifier/UjyqxTpA">uniform resource identifier (URI)</a> scheme for <a href="/facts/Unique_identifier/NMcgc7qX">unique identifiers</a> called tags, defined by RFC 4151 in October 2005. </p><p>The RFC identifies four requirements for tags: </p> <ul><li>Identifiers are likely to be unique across space and time, and come from a practically inexhaustible supply.</li> <li>Identifiers are relatively convenient for humans to mint (create), read, type, remember etc.</li> <li>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.</li> <li>The identifiers are independent of any particular resolution scheme.</li></ul> <p>Tags are used extensively in <a href="/facts/YAML/mAY10H2Y">YAML</a>. </p>