What does the XML syntax with a colon mean?

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The XML syntax, rd:, is a namespace prefix, an abbreviation for a namespace URI.

XML Namespaces

The purpose of XML namespaces is explained clearly in Section 1, Motivation and Summary of the W3C Recommendation: Namespaces in XML 1.0 (Third Edition):

We envision applications of Extensible Markup Language (XML) where a
single XML document may contain elements and attributes (here referred
to as a “markup vocabulary”) that are defined for and used by multiple
software modules. One motivation for this is modularity: if such a
markup vocabulary exists which is well-understood and for which there
is useful software available, it is better to re-use this markup
rather than re-invent it.

Such documents, containing multiple markup vocabularies, pose problems
of recognition and collision. Software modules need to be able to
recognize the elements and attributes which they are designed to
process, even in the face of “collisions” occurring when markup
intended for some other software package uses the same element name or
attribute name.

These considerations require that document constructs should have
names constructed so as to avoid clashes between names from different
markup vocabularies. This specification describes a mechanism, XML
namespaces, which accomplishes this by assigning expanded names
to elements and attributes.

So, in RDL as with in other standards efforts, namespaces allow the creation of a vocabulary without worry of naming collisions with other vocabularies.

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