When the test signal is positive, this means that the voltage and current are positive so the signature's trace is on the right hand side of the LCD display. When the test signal is negative, the voltage and current are negative so the trace is in the left hand side of the display.
Turn on the Tracker 2800 and observe the LCD display. With nothing connected to its test terminals, the display trace is a horizontal line (RL=∞) as shown in figure 2-9.