A zoomable value whic specifies the width of the border (a.k.a. casing) when drawing line (TODO). You can specify the width either as a percentage of the LineWidth (TODO) or in absolute size (in pixels).
define border-width : 25%
specifies that the line border width will be 25% of the line width.
define border-width : 8:1;16:3
specifies that the line border width will have width of 1 pixel up to the zoom level of 8 and then grow to the width of 3 pixels at the zoom level of 16 (and above).