Private Use Characters
The UCS includes 137,468 code points for private use in three different ranges, each called a Private Use Area (PUA). The Unicode standard recognizes code points within PUAs as legitimate Unicode character codes, but does not assign them any (abstract) character. Instead, individuals, organizations and software vendors are free to use them as they see fit. Within closed systems, characters in the PUA can operate unambiguously, allowing such systems to represent characters or glyphs not defined in Unicode. In public systems their use is more problematic, since there is no registry and no way to prevent several organizations from adopting the same code points for different purposes. One example of such a conflict is Apple’s use of U+F8FF for the Apple logo, versus the ConScript Unicode Registry’s use of U+F8FF as klingon mummification glyph in the Klingon script.
The Basic Multilingual Plane includes a PUA in the range from U+E000 to U+F8FF (6,400 code locations). Plane Fifteen and Plane Sixteen have a PUAs that consist of all but their final two code locations, which are designated non-characters. The PUA in Plane Fifteen is the range from U+F0000 to U+FFFFD (65,534 code locations). The PUA in Plane Sixteen is the range from U+100000 to U+10FFFD (65,534 code locations).
PUAs are a concept inherited from certain Asian encoding systems. These systems had private use areas to encode what the Japanese call gaiji (rare characters not normally found in fonts) in application-specific ways.
Schemes and initiatives that use the PUA include:
Read more about this topic: Mapping Of Unicode Characters
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