Real-world text usually contains codepoints from a mixture of different Unicode scripts (including punctuation, numbers, symbols, white-space characters, and other codepoints that do not belong to any script). Real-world text may also be marked up with formatting that changes font properties (including the font, font style, and font size).

For shaping purposes, all real-world text streams must be first segmented into runs that have a uniform set of properties.

In particular, shaping models always assume that every codepoint in a text run has the same direction, script tag, and language tag.