UB zone vs UD zone building land?

[Daft Doctor]

Hi, I have been looking at a few plots of land, some are designated UB on our PLU and some are UD.  I did ask the agent to explain the difference from a practical construction point of view but I didn't get a full answer, apart from that UD land has a COS of 0.2 and that the COS UB is worked out differently.  If anyone can enlighten me a little I would be grateful, including whether whether the building rules covering such land determined locally or nationally.  Many thanks in anticipation.