The Baitrunner D gives you the confidence of a legendary drivetrain and auto-return Baitrunner feature as well as better castability, better line lay, less backlash, wind knots or tangles, less friction on the line, larger, more comfortable grips, a higher max drag as well as a wider range of Baitrunner settings than its predecessors.
 
See Manufacturer's Spec https://fish.shimano.com/content/fish/northamerica/us/en/homepage/Shimano_Product_Page.P-BAITRUNNER_D.html