Jamieson, Fausset and Brown on 1 Chronicles 27:31

31. rulers of the substance that was king David's--How and when the king acquired these demesnes and this variety of property--whether it was partly by conquests, or partly by confiscation, or by his own active cultivation of waste lands--is not said. It was probably in all these ways. The management of the king's private possessions was divided into twelve parts, like his public affairs and the revenue derived from all these sources mentioned must have been very large.