Jeff,
D simply indicates the number of driving wheels as 6.
10 is the total number of wheels and with no suffix it is a 4-6-0
The 1/4 suffix indicates the presence of both an engine truck and a trailing truck, and the specific 1/4 fraction indicates a 2 wheel trailing truck as well, thus the engine is a 2-6-2.
An engine with and engine truck and a 4 wheel trailing truck would have the fraction 2/4 after the total number of wheels, while an engine with only a trailing truck would have the fraction 1/3.