The RJ203 is an RJ45 jack/magnetics front-end add-on module designed to work with the Davicom DM9000B Ethernet controller.
The RJ203's patented design allows you to put other components right under it, and thus minimize the required host PCB size. The bottom portion of the device's front face is translucent. Place status LEDs directly on the host PCB under the RJ203, and they will be visible through the front face of the module.
You can also use the RJ203 in combination with the EM1206 and EM203 modules, which fit right under the RJ203.