The mdoc(7) manpage says about .Bl that The -width and -offset arguments accept scaling widths as described in roff(7) or use the length of the given string.
The words "width" or "offset" do not appear anywhere in roff(7). A description of the often seen 'Ds' is given in the .Bd section, but again it points to roff(7) for the complete description, which is not there. Is that meant to be another roff-related manpage from base? Or groff(7)? Jan