documentclass[12pt,a4paper]{article}
usepackage[french,german]{babel}
usepackage[utf8]{inputenc}
usepackage[T1]{fontenc}
usepackage[style=biblatex-swiss-legal-general,backend=biber,autolang=other,jurisdictionindex=true,commentarystyle=styleA]{biblatex}
addbibresource{Referencingtest.bib}

newbibmacro{customa:styleD}{% <-- this is actually a test, not having any idea
bibsstring{thefield{series}}%    how it will look like, being unable to make it appear
printtext{-}%                     on the pdf result
printnames{labelname}%
addcommaaddspace%
printfield{titleaddon}addspace%
printfield{part}%
}%

DeclareBibliographyOption{commentarystyle}{%
 ifcsdef{blx@opt@commentarystyle@#1}
{blx@commentarystylecsuse{blx@opt@commentarystyle@#1}}
{blx@err@invopt{commentarystyle=#1}{}}}
cslet{blx@opt@commentarystyle@styleA}z@
letblx@opt@commentarystyle@styleB@ne
letblx@opt@commentarystyle@styleCtw@
letblx@opt@commentarystyle@styleD % this is where I'm having trouble.