[format], [input.output] Hallucinated reference to "Unicode Standard, Chapter 3.9 U+FFFD Substitution in Conversion"

In the current standard, we have four references to

Unicode Standard, Chapter 3.9 \ucode{fffd} Substitution in Conversion

Something is wrong here.

• There is a Chapter 5.22 since Unicode 11 which is titled U+FFFD Substitution in Conversion, but it's not Chapter 3.9. This chapter number and title remain unchanged up to and including the latest Unicode 18 draft.
• That Chapter 5.22 used to be called "Best Practice for U+FFFD Substitution" for several standards before Unicode 11, so it's not just that it's been moved from 3.9 to 5.22.
• Chapter 5.22 makes several references to Chapter 3.9 Unicode Encoding Forms (which also includes the definition of "maximal subpart"), but Chapter 3.9 has never been called "U+FFFD Substitution in Conversion".

Basically, we need to decide for the existing references whether they are intended to refer to §3.9 or to §5.22 and how to phrase that.

Search the current standard for the four references to the quoted Unicode citation and inspect their surrounding context. Determine whether each reference targets §3.9 or §5.22, then update the wording so all citations identify the intended section and title.