For most purposes, you can think of “IPL source A” as being something like “the last known good configuration”. And you can think of “IPL source B” as being the current configuration.
Now, that’s not really what the two sides are. It’s just a parallel way of seeing how other systems have similar elements.
The two areas refer to two versions of the low-level instructions that make up the virtual machine that the operating system runs on. As fixes for the VM are made available from IBM, they are distributed as PTFs for the Licensed Internal Code (LIC).
PTFs can be applied in a ‘temporary’ status. That allows them to be run to be sure they work in your environment, yet it’s easy to back them out if problems appear. Once a PTF is changed to ‘permanent’ status, there’s no way to back it out.
The A-side is the version of the VM that has all LIC PTFs permanently applied. The B-side is the version of the VM that has some temporary PTFs. Because the temporary PTFs have not yet been incorporated into the A-side, that side remains available for IPLs if anything goes wrong with the B-side.
When LIC PTFs are eventually permanently applied, there is no difference between the two sides.
For most systems, the B-side is all that is used. There is rarely (if ever) a need to switch deliberately to the A-side. It’s just there in case it’s needed.