Conceptually, MathML has two parts: presentation and content. They form a single language and it is easy to mix them, but they focus on different goals: the presentation part expresses the two-dimensional look of a formula and is most suitable for publications, such as articles and books; the content part focuses on the semantics and is most suitable as the interchange format between applications for engineering, statistics, etc.
MathML “presentation” markup is typically inserted in XML-based document formats, including XHTML (and soon HTML) mixed with the document's text. MathML “content” markup is more often used on its own.
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