Pots are all basically the same thing. There are very minor changes in each that make them more specialized at something than others. A and B pots are most common becuase they are specialized pots. A pots are generally used as tone pots and B's are used as volume pots. They each do the specific task better than the other. However, D pots are more of a rarity. They're more of a "middle-of-the-road" pot. They're the multi purpose pots. They won't outperform an A pot as a tone pot but will as a volume pot. And vice-versa for the B pots. So, if the diagram has the D pot as tone pot, you can use an A... if it's a volume pot, you can use a B. I would recommend it that way anyway. 90% of guitar parts stores don't even carry D500k pots.