Thank You Péter,
your workaround works well.
But I found another solution:
AC_GLOB_ELEVATION works astonishingly in the marker and gives back the sill height of the window,
although this parameter is not created as paraneter in the window object (so ASSOCLP_PARVALUE does not work here).
If AC_GLOB_ELEVATION is as parameter in the marker, I can compare AC_GLOB_ELEVATION with AC_Sill_To_Curr_Story,
and if the 2nd one is bigger than the 1st one, I substract the story heigth of the sory below.
GDL Object Developer
MAC OSx 10.12