\enabletrackers[marks.get,marks.detail]

\setupheadertexts
%     [{(\getmarking[section]) / (\fetchallmarks[section])}]
%     [{\getmarking[section][previous]}]
    [{\getmarking[section][top]}]
%     [{\getmarking[section][first]}]
%     [{\getmarking[section][first:nocheck]}]

\starttext
%     \chapter[title={T},marking={T}] % \dorecurse{14}{\input tufte}
    \section[title={A},marking={A}]
%     \dorecurse{10}{\input tufte}
    \dorecurse{5}{\input tufte}
    \section[title={B},marking={B}]
%     \dorecurse{10}{\input tufte}
    \section[title={C},marking={C}]
%     \section[title={D},marking={D}]
%     \dorecurse{10}{\input tufte}
\stoptext
