This has been asked, or brushed by before, but not really answered, for instance, here: SMD LED footprint
I still don't know what the 3rd footprint pad for some SMD LEDs (like the Vishay VSMG10850; the red bit in the image) is for, do I need to add one on the board, and if so, do I leave it unconnected, connect it to the GND for instance, or what?
Some of the explanations mentioned it might be for heat dissipation. If so, how should it be connected to anything larger than the small pad for actually dissipating some of the heat? Another guess is that it has to do with just mechanical attachment to the board, especially for automated layout hardware.