On the Resmed Auto VPAPs, that PS is going to be the constant difference IPAP and EPAP.
As the machine adjusts the pressure up and down, the IPAP and EPAP will both be adjusted at the same time and by the same amount.
It may help to consider a few examples: Let's say that we have Min EPE = 8 and MAX IPAP = 16. Now lets consider what happens with different PS levels:
With PS = 2, we have the following ranges 8 <= EPAP <= 14 and 10 <= IPAP <= 16
With PS = 3, we have the following ranges 8 <= EPAP <= 13 and 11 <= IPAP <= 16
With PS = 4, we have the following ranges 8 <= EPAP <= 12 and 12 <= IPAP <= 16
With PS = 5, we have the following ranges 8 <= EPAP <= 11 and 13 <= IPAP <= 16
With PS = 6, we have the following ranges 8 <= EPAP <= 10 and 14 <= IPAP <= 16
So in "picking" a PS, what's important?
Well comfort is one consideration. The basic premise of bi-level is that providing a sufficiently large enough drop in pressure on exhale should make the machine more comfortable to use. But if the drop is too large, that can cause comfort (and leak) problems for some individuals. Really large PS settings may leave you feeling as machine is blasting air down you gut with every inhalation, which may not be very sleep inducing.
But therapy needs are also important: That EPAP needs to be able to increase far enough to deal with nasty clusters of OAs and significant snoring during the times when you are running at max pressure. In other words, the PS needs to be set so that Max EPAP = Max IPAP - PS should be at least 1-2cm above your 95% EPAP. And the initial IPAP = Min EPAP + PS needs to be high enough for you to not feel like you are air-starved. And that initial IPAP should be high enough to deal with most of you flow limitations and hypopneas. In other words, Min EPAP + PS should probably be within 2-4cm of your 95% IPAP.