This pane lets you customize the different playing styles/parts of the instrument. Each one can be positioned at a specific keys range and set to specific parameters. You can move them to another region of the keyboard, resize them and even drag them on other rows. You can also add more styles by dragging them from the ones shown below.
Clicking on a style selects it and you can then optionally set specific parameters to it. These will override the global settings of the voice.