n

    The subpage letter that will be appended to the number of the
    next page. You can specify the letter itself (A) or you can
    specify a number corresponding (in order) to the letter that
    the subpage will have. For example, 1=A, 26=Z, 27=AA, 28=AB.

 +n

    Specifies how many alphabetically ordered letters past the
    current subpage letter the next subpage letter will be. For
    example, if the current subpage is page 3-12E, then .NUMBER
    SUBPAGE +2 will cause the next subpage to be numbered 3-12G.

 -n

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