Jacketed and Cored Valve Bodies

Drilled. material removed to allow steam passage?

That's my guess.

It probably refers to one made from a casting that is cored like an Engine Block is cored for liquid cooling.

The valve would be designed as a double walled vessel where the valve compartment is in the interior made by a core or cores and the steam jacket passages would be accomplished by a second set of cores. The inner wall of the second set or cores creates the walls of the valve that the fluid is passing through while the outer most wall would contain the heating medium which could be either steam, hot water or other fluid.

These are used extensively in plants where molten sulfur is produced or utilized or where a minimum temperature must be kept to prevent the solidification of the product.

Jacketed would be more like a tracing where you wrap a coil around the valve and then insulate it to keep in the heat, much like you do with water pipes that can freeze up in the cold with an electric tracing wires which you then wrap with fiberglass insulation.