The mark is a sign of failed decoding
A computer stores text as numbers, not as pictures of letters. To show those numbers as readable characters, a program has to interpret them using an agreed encoding—a system that maps numbers to letters and symbols. If the incoming data does not make sense under the encoding being used, the program may show �, the Unicode replacement character, often drawn as a black diamond with a question mark.
This can happen when text is saved in one encoding but opened as though it used another, or when a file or message contains damaged or incomplete data. The program cannot reliably identify the intended character, so it substitutes a visible warning instead of displaying a character it cannot decode.
The original character may already be lost
The mark is not usually a clue to one particular missing letter. It means the software could not recover a character at that point in the text. If the replacement character was inserted during decoding and the damaged text was then saved, the original data may no longer be available to restore; reopening it with a different encoding will not necessarily bring the character back.
That makes the symbol more than a display glitch: it can reveal where text crossed a boundary—between programs, files, or systems—that interpreted its bytes differently. A few diamonds in a message may be cosmetic, but in a name, address, or record, they can hide information that matters.
