So partial pressure and total pressure are completely different. Partial pressure relates to the pressure of a target gas and total pressure is the amount of pressure in an atmosphere. So the unit can withstand and work in a pressure range of 8.7 to 17.4 psia. (this is just psi but atmospheric psi).
Then the sensor unit has a partial pressure range of 5.8 so the sensor measures the oxygen molecules or content up to 5.8psi. The reason it's in PSI instead of % is because psi can help us identify the effects of O2 molecules at pressure. So PPO2 lets us analyse the molecules at pressure of which can affect the human body.
Converting PPO2 to %:
%O2 = [mBar.pp O2 / Absolute pressure (mBar)] x 100%
Absolute Pressure (mBar) = (MSW +10) x 100
So %O2 = [mBar.pp O2] / [MSW + 10]
MSW= 1.46PSI