What does "PC DTR" (pin 4) or "RING" (pin 9) mean and are they needed for this to work?

RS232 Pin Assignments (DB9 PC signal set)

Pin 1 Received Line Signal Detector (Data Carrier Detect)
Pin 2 Received Data
Pin 3 Transmit Data
Pin 4 Data Terminal Ready
Pin 5 Signal Ground
Pin 6 Data Set Ready
Pin 7 Request To Send
Pin 8 Clear To Send
Pin 9 Ring Indicator

Of these, the empeg only makes proper use of 2, 3, and 5. Connecting the others will confuse whatever device you're connected to.