just what the subject says. It seems an important feature to me but
I'm unable to find any reference to it in
https://orgmode.org/manual/Macro-replacement.html.

Strangely I do find it here:

https://orgmode.org/manual/Macro-Replacement.html#Macro-Replacement

The only difference being that "replacement" is capitalized.

I had been having this feeling of accessing to different versions of the documentation for some time now. Is there any double indexing of the docs? I reached both pages from Google.