In barrel racing, "1D", "2D", "3D" and "4D" are terms that denote the first, second, third and fourth divisions. Divisions are determined by taking the fastest time ran by a horse on a specific track and marking that as "1D," then setting the following divisions by adding set segments of time to the fastest time posted.
For instance, if a horse runs a track in 17 seconds, then 17 seconds will be the fastest time of division one. It is different for every association, but normally the second division will start with times that are half a second slower than 17 seconds, the third division will start with times that are 1 second slower than 17 seconds, and so on. This system guarantees that everyone only competes against members of their own division instead of competing with the fastest time overall.