What the CPU can address, and what is actually installed are two different things.
Yes, the memory map is arranged such that it can address 1GB of DDR. That doesn't mean that you have to have 1GB of DDR attached, only that there is room for 1GB of DDR.
The "PL" is the "Programmable Logic" of the chip. The "PL" areas are set aside specifically for interfacing with the programmable areas of the chip. What can you store there? Well, nothing really, it's not intended for that. You could program the programmable logic to give you some memory mapped registers in that area and store values in there, but that sort of thing is meant to be used for controlling the programmed logic, not for just storing arbitrary values.
The 4GB memory space is basically split up into four 1GB chunks. They are:
- DDR access
- PL port 0
- PL port 1
- CPU and interfacing control
Each section can be further subdivided, but those are the main blocks, each identified by the upper two bits of the address value.