D1/D2/D3 Levels
There are three D levels:  D1, D2 and D3.  The D-Levels offer an introduction to the fun and challenge of riding, establishing a foundation of safety habits andknowledge of the daily care of a mount and related equipment. The D-level Horse Management  focuses on acquiring the knowledge and skills related to care and ground handling of the mount.
The D level-member may also learn and be evaluated on riding independently on the flat, with control, maintaining a reasonably secure position at the walk, trot and canter. In addition, the D-level members may choose to learn and be evaluated while riding their mounts over low fences. All D certificates are awarded at the club level.
The US Pony Club Chart of Learning