The PSI or Bar rating is the force that the particular pump will pressurise the water up to.
The flow rate is the amount of water pushed out per minute at full open flow.
You could have a 30 lpm pump that has only 30 psi or conversly a 1 lpm pump that has 160 psi.
So why quote a figure of a given pump 40psi(shurflo backpack) so its not so good as an 80psi one(wcw backpack).
I was also told by a supplier that my 100 psi shurflo pump when used in conjunction with a varistream that has its internal blue dial set on maximum, will give a pressure way way in excess of 100psi, so the psi rating is what I originally said and thats the pressure that the switch kicks in cutting the pump off.
I'm sure Alex mentioned he uses a 100psi pump but with a 60 psi pressure switch