\definestructureresetset[default][0,0][1]

\setuphead[part][sectionresetset=default]
\setuphead[part][resetnumber=no,placehead=yes,conversion=R]
\setuphead[chapter,section,subsection][resetnumber=no,placehead=yes,sectionsegments=2:100]
\starttext
\part{part one}
\chapter{chapter one}
\section{section one}
\section{section two}
\chapter{chapter two}
\section{section one}
\section{section two}
\part{part two}
\chapter{chapter three}
\section{section one}
\section{section two}
\chapter{chapter four}
\section{section one}
\section{section two}
\stoptext

