This only has an effect on certain line styles which are not symmetrical (for example, Border.LineStyle.ThinThickSmallGap). Reversing the right and bottom graphics of certain line styles can create a "3D" effect. CharacterRuns have only one border. Paragraphs have different border parts. For borders around characterRuns, Reverse tells you whether or not the right and bottom sides of the border have reversed graphics. For borders around paragraphs, it is necessary to explicitly reverse the right and bottom sides.
Reverse defaults to false which means that by default borders will appear "boxed".
MS Word equivalent: Format menu > Borders and Shading... > Borders tab > Setting: > Box