How did the parity check DRAM chips work? In particular, is there a reason for the gap in the motherboard (IBM calls this system board) between the parity check DRAM and the "normal" DRAM above it?

There is a also a gap in the 8kB ROM sockets to the left of the DRAM sockets, between the U28 socket and the U29 socket above it. This makes the board tidier and aligns the U28 socket with the RAM parity chips to the right. Is there a reason for this gap?

A lesser question is whether there are any products that utilised the ROM expansion socket (labeled U28)?