It seams that my context does not know definesectionalternative\definesectionalternative[mysection][renderingsetup=mysection]
\startsetups[mysection]\vbox\bgroup\headsetupspacing\strut\headnumbercontent\par\begstrut\headtextcontent\endstrut\egroup\stopsetups
\setuplabeltext[en][section={}{Section]
\setuphead[section][alternative=mysection]