DAR stands for Dressed All Round, timber that has been machined smooth and to a consistent square section on all four faces, rather than left rough-sawn straight off the saw. A picket or rail callout like "70x19 DAR" means a board 70 mm wide by 19 mm thick, dressed on every face, so it paints and fixes cleanly with no rough grain to catch a hand or a coat of finish.
Dressing takes a little material off a rough-sawn size, which is why a nominal 70x19 DAR board is not exactly 70 by 19 once you measure it with calipers, the same way a nominal 90x45 stud is not exactly that size either. Trade sections are quoted by their DAR (finished) size, and that is the number CAD60 draws and cuts to.
Treated pine DAR is the standard timber picket and rail section: 65x19, 70x19, or 88x19 for pickets, and 70x35 up to 90x45 for rails and gate ledges. Hardwood DAR runs the same nominal sizes for a heavier, harder-wearing fence or gate. CAD60 labels every timber member's section as "DAR TIMBER" or "DAR HARDWOOD" on the drawing and cutting list, distinct from the RHS and SHS callouts on every welded aluminium or steel model in the catalogue.