"Contended memory space, on the other hand, is shared between the ULA and the Z80 and the ULA has higher priority. Contended memory occupies addresses 0x4000..0x7FFF of the Z80 memory map. This is the first 16 KB of RAM in the 48 KB machine but the entire RAM of the 16 KB machine."
Okay, I can sort of understand that in a scenario where the 48K machine uses three banks of 16kbit RAM chips, so just one bank is shared.
But as I understand it, later versions of the 48K Spectrum used a single bank of 64kbit RAM chips (so actually 64K RAM of which 16K was unused). How is that consistent with the possibility of only 16K being contended?