U+FFF9 Interlinear Annotation Anchor

The Wikipedia has the following information about this codepoint:

Many Unicode characters are used to control the interpretation or display of text, but these characters themselves have no visual or spatial representation. For example, the null character (U+0000 NULL) is used in C-programming application environments to indicate the end of a string of characters. In this way, these programs only require a single starting memory address for a string (as opposed to a starting address and a length), since the string ends once the program reads the null character.

In the narrowest sense, a control code is a character with the general category Cc, which comprises the C0 and C1 control codes, a concept defined in ISO/IEC 2022 and inherited by Unicode, with the most common set being defined in ISO/IEC 6429. Control codes are handled distinctly from ordinary Unicode characters, for example, by not being assigned character names (although they are assigned normative formal aliases). In a broader sense, other non-printing format characters, such as those used in bidirectional text, are also referred to as control characters by software; these are mostly assigned to the general category Cf (format), used for format effectors introduced and defined by Unicode itself.