The .DISPLAY LEVELS command allows you to specify the form that
    sequential numbering (or lettering) of section headers will
    take. You can control the form of individual numbers within a
    section number for a header (that is, those numbers preceding
    or following a dot). This command does not change any values; it
    affects only the way the values are displayed.(See also .HEADER
    LEVEL, .NUMBER LEVEL, and .STYLE HEADERS.)

    Default Header Level Numbering

    Command            Nonchapter  Chapter n   Appendix A

    .HEADER LEVEL 1    1           n.1         A.1
    .HEADER LEVEL 2    1.1         n.1.1       A.1.1
    .HEADER LEVEL 3    1.1.1       n.1.1.1     A.1.1.1

    Format

      .DISPLAY LEVELS  [y1] [,y2]...[,y6]

    Abreviated format

      .DHL  [y1][,y2]...[,y6]