\useMPlibrary[dum]

\setuppapersize[A4][A3]

\setuplayout[location=middle]

\showframe

\starttext

    \setupcaption [figure][]
    \placefloat[][]{default}{\externalfigure[dummy][]}

    \setupcaption [figure][location={rightmargin,low}]
    \placefloat[][]{right}{\externalfigure[dummy][]}

    \setupcaption [figure][location=rightmargin]
    \placefloat[][]{right}{\externalfigure[dummy][]}

\stoptext
