Where are the traces connected to RESET-EN leading?
This is the DTR output of the ATmega16u2 which is connected to RESET pin of the ATmega2560 for reset by avrdude over DTR signal before upload of the sketch. There is a trace by default. Cutting the trace disables the auto-reset. Soldering a bridge on  RESET-EN pads re-enables the auto-reset.



the schematics part you quoted in question misses the long unconnected line for the RESET label. It leads here: