XmTabSetValue man page
XmTabSetValue — A convenience function that sets a tab stop
#include <Xm/Xm.h> void XmTabSetValue( XmTab tab, float value);
XmTabSetValue sets the value field of the XmTab structure associated with tab.
Specifies the tab to set the value of.
Specifies the floating point number which represents the distance that the rendering of the XmString segment associated with tab will be offset. The offset is from either the beginning of the rendering or from the previous tab stop, depending on the setting for the offset model. The distance depends on the tab's unit type. Note that negative values are not permitted, and that if a tab stop would cause text to overlap, the x position for the segment is set immediately after the end of the previous segment.
See also the Motif Programmer's Guide for more information about tabs and tab lists. XmTab(3).