Hi,I would like to add USB functionality to one of my PIC projects and I thought the best way of doing that would be to 'borrow' the USB bits of the NG circuit diagram I do have a few questions..
RTS and DTR are two of the hardware handshaking lines for the serial communication. We've been trying to figure out a way to use them to automatically reset the Arduino board when you upload a new sketch (so you don't to press the reset button), which is why the resistors are shown on the schematic. In fact, those resistors aren't mounted and you probably don't want to include them (you don't want those lines connected to reset). The X3 that you've circled is just a group of 4 unused pads on the Arduino board - those signals aren't used for anything (and aren't connected together), but we wanted to break them out in case someone found a use for them.