Private Use Zone

The Private Use Areas (PUA) in Unicode are three ranges of code points (U+E000–U+F8FF in the BMP, and in planes 15 and 16) that, by definition, will not be assigned characters by the Unicode Consortium. The code points in these areas cannot be considered as standardized characters in Unicode itself. They are intentionally left undefined so that third parties may define their own characters without conflicting with Unicode Consortium assignments.

Does it mean that you are the one who can decide what to put there? I'm not sure. In any case, the Unicode Stability Policy suggests that the Private Use Areas remain allocated for the purpose under discussion in all future Unicode versions. However, it is known that interpretation may be determined by private agreement among cooperating users.

Assignments to Private Use Area characters need not be „private“ in the sense of strictly internal to an organisation; a number of assignment schemes have been published by several organisations. Such publication may include a font that supports the definition (showing the glyphs), and software making use of the private-use characters (e.g. a graphics character for a „print document“ function). By definition, multiple private parties may assign different characters to the same code point, with the consequence that a user may see one private character from an installed font where a different one was intended.