n

    The character that specifies what the next appendix letter will
    be. You can specify the letter itself (A) or you can specify a
    number corresponding (in order) to the letter that will identify
    the appendix. For example, 1=A, 26=Z, 27=AA, 28=AB.

 +n

    Specifies how many alphabetically ordered letters past the
    current appendix letter the next .APPENDIX character will be.
    For example, if the current appendix is APPENDIX B, then .NUMBER
    APPENDIX +2 will cause the next .APPENDIX to produce APPENDIX D.

 -n

    Specifies how many alphabetically ordered letters before the
    current appendix letter the next .APPENDIX letter will be.