C99/C11 mention several times the Appendix to IEC 60559. At first sight, it seems it would be Annex F (IEC 60559 floating-point arithmetic), but it is somewhat confusing to see definitions such as this one:
F.10.8.1 The copysign functions
copysign is specified in the Appendix to IEC 60559.
First, there is a difference between Annex and Appendix, even though they are mostly synonyms. Is this relevant?
Secondly, if Annex F is such Appendix, why does it refer to itself in such a recursive manner, without giving the actual definition of function copysign? Shouldn't it say instead "copysign is specified in IEC 60559"?
Finally, I considered interpreting Appendix to IEC 60559 as "an annex to the standard IEC 60559", that would be available in IEC 60559 itself (in the same way as Annex F is present in the C99/C11 standards), but I could not find it in the standard. The preview available at ISO's website indicates only 2 annexes, Annex A - Bibliography, and Annex B - Program debugging support, which are both unrelated to the definitions mentioned in the C standards.
The C standard has an annex F. The IEC 60599 has an appendix.
copysign is mentioned in several places in throughout the C standard.
The C standards annex F discusses the entirety of the IEC 60559-1989 (with regard to the usage in C), which does include mention of copysign.
The date (i.e. 1989) of the IEC 60559 document is important here as the layout of the document is changed significantly in the current version that the ISO website is likely to be selling! Both C99 and C11 reference the 1989 version.
In annex F of the C standard the copysign function of IEC 60559 is specifically mentioned. Within that document (i.e. IEC 60559-1989), it is detailed within the appendix.
Thus:
F.10.8.1 The copysign functions
copysign is specified in the Appendix to IEC 60559.
is part of part 10 of the annex F of the C standard that tells you that copysign is specified within the appendix of IEC 60559.
Within the 1989 version of IEC 60559 (well actually I'm looking at IEEE 754-1985 but the layout should match the 1989 IEC document), copysign is mentioned in the section entitled "Appendix".
Within a more recent version of IEC 60559 (I have IEEE 754-2008 but the layout should match the 2011 IEC document), copysign is mentioned in sections 5.5.1, 6.3 and B.3