Sets or returns an int object that represents the number of heading levels that will be displayed by Word in the outline view for this document. This can be set to a number between 1 and 9.

public int DisplayedOutlineLevels{ get; set; }
Public Property DisplayedOutlineLevels() As Integer

This option is set to 9 (all levels) by default.

MS Word equivalent: Outlining toolbar > Show Level

          //--- Return DisplayOutlineLevels
          int displayOutlineLevels = viewSettings.DisplayOutlineLevels;

          //--- Set ZoomType
          viewSettings.DisplayOutlineLevels = 4;
        
          '--- Return DisplayOutlineLevels
          Dim displayOutlineLevels As Integer = viewSettings.DisplayOutlineLevels

          '--- Set DisplayOutlineLevels
          viewSettings.DisplayOutlineLevels = 4