What's the secondary expansion vessel for?

Potterton Powermax HE 150

I'm going to power off my boiler and replace my expansion vessel this evening (Fig 18 on page 29 of the PowermaxHE_Installation_Service.pdf document - page attached) but what is the SECONDARY expansion vessel for? (Fig 11 on page 19 of the document - page attached).

I believe my secondary vessel is also under-pressured so need to pump it up to 36-40 psi (2.5 - 2.7 bar).

I'm going to be draining ALL of the radiators first (we have a couple that are leaking from the bottom of the TRV and need repairing).

I want to pump up both expansion vessels to the correct pressure before refilling the system.

Just curious as to what the secondary expansion vessel is for.

I assume I have to turn off the 'Stop Valve' in the 'Inlet Control Group' before pumping up the second vessel?

Cheers.

Alex