Different line styles are available for bar lines, for lines splitting cells vertically or horizontally, and for extra column borders. The styles are a combination of a width value (Regular, Medium, Thick) and an appearance value (Sold, Dashed, Dotted).
The regular line width is determined by the settings in the Staff Borders dialog for inside vertical and horizontal lines. The medium and thick styles create a line three times and six times the regular width, respectively.