The following code provides a simple example usage for \definemixedcolumns:
%\setupcolors[state=start] % This is only necessary in older ConTeXt mkii.
\hyphenation{sig-ni-fi-cant-ly} % for correct line formatting
\startuseMPgraphic{StyleVerticalRule}
draw OverlayBox;
\stopuseMPgraphic
\defineoverlay[StyleVerticalRule][\useMPgraphic{StyleVerticalRule}]
\definemixedcolumns
[sectioncolumns]
[n=2, separator=rule, rulecolor=darkgreen, rulethickness=3pt, balance=yes]
% Horizontal line below the section header.
\defineframed[sectionframed]
[frame=off, bottomframe=on, framecolor=darkgreen,
rulethickness=1pt, width=local, align=normal, % to get a vbox
]
\define[2]\StyleSection{\sectionframed{#2}}
\setuphead
[section]
[command=\StyleSection, after={\startsectioncolumns},
aftersection={\stopsectioncolumns}]
\starttext
\startchapter[title={Chapter}, reference=sec:my-chap,]
\startsection[title={Section}, reference=sec:my-sect,]
\startsubsection[title={Content}, reference=sec:my-cont,]
\input knuth
\input knuth
\stopsubsection
\stopsection
\stopchapter
\stoptext