The surface is dusted with iyerosun, a fine pale powder worked from the wood of the irosun tree, which takes a mark cleanly and can be smoothed flat again by hand. Onto that surface the diviner marks the figure that has come up.
The casting itself is done one of two ways. Either sixteen palm nuts, the ikin, are moved between the hands and the number left over is counted each time, or the divining chain, the opele, is thrown and read from the way its eight halves fall. Either method yields a figure of eight positions, and the diviner records it in the powder as two columns of four, each position marked with a single stroke or a double one. The result is one of the 256 Odu. Only then does the corpus come in: the diviner recites from the verses belonging to that figure, looking for the precedent that matches the case in front of him.
So the tray does not answer anything. The nuts or the chain produce the figure, the memory of the diviner produces the reading, and the tray is where the figure is held still long enough to be worked with. That is a modest job for the most famous object in the tradition, and it is also why the tray travelled. Opon Ifa sit in museum collections all over the world, admired as sculpture, with the one thing that made them useful, a working diviner, absent.