There is some rectangle in the middle of the footprint. That has sx and sy dimensions. That part confused my mind. Is it silkscreen?
It's a thermal pad to improve heat conductivity thus allowing the device to operate with higher electrical powers. Check the data sheet of the actual part.
This is not a silkscreen, but is a thermal pad.
You will find in many ICs which deals with good amount of power / wattage. Thermal pads (also called thermally conductive pad or thermal interface pad) are a pre-formed square or rectangle of solid material (often paraffin wax or silicone based) commonly found on the underside of heatsinks to aid the conduction of heat away from the component being cooled. It helps to decrease the temperature of the surface of IC.