U+FDD1 Noncharacter *

The Wikipedia has the following information about this codepoint:

The Unicode Consortium and the ISO/IEC JTC 1/SC 2/WG 2 jointly collaborate on the list of the characters in the Universal Coded Character Set. The Universal Coded Character Set, most commonly called the Universal Character Set (abbr. UCS, official designation: ISO/IEC 10646), is an international standard to map characters, discrete symbols used in natural language, mathematics, music, and other domains, to unique machine-readable data values. By creating this mapping, the UCS enables computer software vendors to interoperate, and transmit—interchange—UCS-encoded text strings from one to another. Because it is a universal map, it can be used to represent multiple languages at the same time. This avoids the confusion of using multiple legacy character encodings, which can result in the same sequence of codes having multiple interpretations depending on the character encoding in use, resulting in mojibake if the wrong one is chosen.

UCS has a potential capacity of over 1 million characters. Each UCS character is abstractly represented by a code point, an integer between 0 and 1,114,111 (1,114,112 = 2²⁰ + 2¹⁶ or 17 × 2¹⁶ = 0x110000 code points), used to represent each character within the internal logic of text processing software. As of Unicode 16.0, released in September 2024, 299,056 (27%) of these code points are allocated, 155,063 (14%) have been assigned characters, 137,468 (12%) are reserved for private use, 2,048 are used to enable the mechanism of surrogates, and 66 are designated as noncharacters, leaving the remaining 815,056 (73%) unallocated. The number of encoded characters is made up as follows:

• 149,641 graphical characters (some of which do not have a visible glyph, but are still counted as graphical)
• 237 special purpose characters for control and formatting.

ISO maintains the basic mapping of characters from character name to code point. Often, the terms character and code point will be used interchangeably. However, when a distinction is made, a code point refers to the integer of the character: what one might think of as its address. Meanwhile, a character in ISO/IEC 10646 includes the combination of the code point and its name, Unicode adds many other useful properties to the character set, such as block, category, script, and directionality.

In addition to the UCS, the supplementary Unicode Standard, (not a joint project with ISO, but rather a publication of the Unicode Consortium,) provides other implementation details such as:

1. mappings between UCS and other character sets
2. different collations of characters and character strings for different languages
3. an algorithm for laying out bidirectional text ("the BiDi algorithm"), where text on the same line may shift between left-to-right ("LTR") and right-to-left ("RTL")
4. a case-folding algorithm

Computer software end users enter these characters into programs through various input methods, for example, physical keyboards or virtual character palettes.

The UCS can be divided in various ways, such as by plane, block, character category, or character property.