Here are two Training Peaks articles that will provide some good insight into these metrics –
My primary use of Pa:Hr is related to the heart rate drift (AeT) test. This heart rate drift article will provide you some good information. For a given test/workout, a declining Pa:Hr would be good. e.g. When I repeat AeT test on a treadmill with fixed incline and pace, if my Pa:Hr is trending lower then it suggests an improvement in my AeT.
rTSS – as the Training Peaks article states it is a metric to estimate the training load and physiological stress created by the training session. As you progress in your training and improve your aerobic capacity, TSS will tend to go down for a given workout.
Hope this is helpful.