Change biblatex field format only when field has been replaced

Question

In my document, I want to have all abbreviations consisting of consecutive capitals to be set 90 per cent of the font size. I can do that manually in the text. But if it comes to journal abbreviations, I dont know how to achieve this for citations.

A DeclareFieldFormat would not work because the field shortjournal is only used to overwrite the journal field when present. And I dont want to set the journal field 10 per cent smaller in all cases but only in the cases when the full journal title is replaced by shortjournal.

In short: How can I re-format the journal field in citations but only in those cases where they have been replaced by shortjournal, as in my MWE?

I'm compiling with XeLaTex.

M(not yet)WE:

\documentclass{book}
\usepackage{relsize}
\newcommand{\abk}{\textscale{0.9}}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@ARTICLE{AB,
         author    = {Author Beta},
         title     = {Title},
         shortjournal = {ABC},
         journal ={AllButCrash},
         pages = {1-23},
         year={2009}}
@ARTICLE{CD,
         author    = {Changed Director},
         title     = {Title},
         journal ={Boring Texts},
         pages = {23-42},
         year={2019}}
\end{filecontents}

\usepackage[backend=biber,style=verbose-inote,sorting=nyt,bibencoding=utf8,citereset=chapter,citepages=separate,refsection=none,autocite=footnote,isbn=false,doi=false,url=false,eprint=false,date=short]{biblatex}
\addbibresource{\jobname.bib}

\DeclareSourcemap{
  \maps[datatype=bibtex]{
    %Shortjournal berücksichtigen      
      \map[overwrite]{
      \step[fieldsource=shortjournal]
      \step[fieldset=journal,origfieldval]
    }
    }
}

\begin{document}
A reference here.\footcite{AB} All Abbreviations in Capitals -- whom I call \abk{AAIC} -- should be in a font size 10 percent smaller. So that they don't look like THIS.\footcite{CD}

\end{document}

Answer 1

The data substitution you are attempting leaves no trace that such substitution took place. So, without further information, on biblatex's side we cannot know if a source map data change happened. But we could add such information by means of field annotations.

So, we could add one such annotation in the same map with:

\DeclareSourcemap{
  \maps[datatype=bibtex]{
    % Shortjournal berücksichtigen
    \map[overwrite]{
      \step[fieldsource=shortjournal, final]
      \step[fieldset=journaltitle, origfieldval]
      \step[fieldset=journaltitle+an, fieldvalue={=shortened}]
    }
  }
}

With that, the journaltitle field, when it has been substituted by shortjorunal in the source map, will also receive a field annotation =shortened, which we can then use as a conditional in the formatting directive:

\DeclareFieldFormat{journaltitle}{%
  \iffieldannotation[journaltitle]{shortened}{%
    \mkbibemph{\abk{#1}}%
  }{%
    \mkbibemph{#1}%
  }%
}

In full:

\documentclass{book}

\usepackage{relsize}
\newcommand{\abk}{\textscale{0.9}}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@Article{AB,
  author       = {Author Beta},
  title        = {Title},
  shortjournal = {ABC},
  journaltitle = {AllButCrash},
  pages        = {1-23},
  year         = {2009},
}
@Article{CD,
  author       = {Changed Director},
  title        = {Title},
  journaltitle = {Boring Texts},
  pages        = {23-42},
  year         = {2019},
}
\end{filecontents}

\usepackage[
  backend=biber,
  style=verbose-inote,
  sorting=nyt,
  bibencoding=utf8,
  citereset=chapter,
  citepages=separate,
  refsection=none,
  autocite=footnote,
  isbn=false,
  doi=false,
  url=false,
  eprint=false,
  date=short,
  ]{biblatex}
\addbibresource{\jobname.bib}

\DeclareSourcemap{
  \maps[datatype=bibtex]{
    % Shortjournal berücksichtigen
    \map[overwrite]{
      \step[fieldsource=shortjournal, final]
      \step[fieldset=journaltitle, origfieldval]
      \step[fieldset=journaltitle+an, fieldvalue={=shortened}]
    }
  }
}

\DeclareFieldFormat{journaltitle}{%
  \iffieldannotation[journaltitle]{shortened}{%
    \mkbibemph{\abk{#1}}%
  }{%
    \mkbibemph{#1}%
  }%
}

\begin{document}
A reference here.\footcite{AB} All Abbreviations in Capitals -- whom I call
\abk{AAIC} -- should be in a font size 10 percent smaller. So that they don't
look like THIS.\footcite{CD}

\end{document}

Another alternative would be not to do any data substitution, and handle the conditional directly in the relevant bibmacro, in this case journal:

\documentclass{book}

\usepackage{relsize}
\newcommand{\abk}{\textscale{0.9}}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@Article{AB,
  author       = {Author Beta},
  title        = {Title},
  shortjournal = {ABC},
  journaltitle = {AllButCrash},
  pages        = {1-23},
  year         = {2009},
}
@Article{CD,
  author       = {Changed Director},
  title        = {Title},
  journaltitle = {Boring Texts},
  pages        = {23-42},
  year         = {2019},
}
\end{filecontents}

\usepackage[
  backend=biber,
  style=verbose-inote,
  sorting=nyt,
  bibencoding=utf8,
  citereset=chapter,
  citepages=separate,
  refsection=none,
  autocite=footnote,
  isbn=false,
  doi=false,
  url=false,
  eprint=false,
  date=short,
  ]{biblatex}
\addbibresource{\jobname.bib}

\renewbibmacro*{journal}{%
  \iffieldundef{shortjournal}
    {\ifboolexpr{
      test {\iffieldundef{journaltitle}}
      and
      test {\iffieldundef{journalsubtitle}}
     }
       {}
       {\printtext[journaltitle]{%
          \printfield[titlecase]{journaltitle}%
          \setunit{\subtitlepunct}%
          \printfield[titlecase]{journalsubtitle}}}}
    {\printfield{shortjournal}}}

\DeclareFieldFormat{shortjournal}{\mkbibemph{\abk{#1}}}

\begin{document}
A reference here.\footcite{AB} All Abbreviations in Capitals -- whom I call
\abk{AAIC} -- should be in a font size 10 percent smaller. So that they don't
look like THIS.\footcite{CD}

\end{document}