Polarity Ratio represents the total amount of strain above threshold power to the total amount strain below threshold power. It looks at your work/strain allocation ratios for the strain you accumulate during training to establish the follow ratio:

low strain : (peak strain + high strain)

For example, according to this chart, representing the work allocation for a 4 minute Puncheur Focus with Pure Specificity rating, the polarity ratio is 70:30.