RPE measures feelings of effort, strain, discomfort, and/or fatigue experienced during both aerobic and resistance training, on a scale of 1-10, with 1 being very easy and 10 being extremely hard.
VeloPro uses the Coggan scale for Heart Rate (HR) and the Borg 10-Point scale for RPE (Rate of Perceived Exertion). The best way to figure out HR is to do an all-out 20 mins effort. The average HR for this effort is your Lactate Threshold HR, from which your zones are calculated.
British Cycling have a calculator to help you with this.
Additionally, see the Borg scale below.