Addressable space
Depends on the processor capabilities, which can be around 36~40 bits for recent Intel processors. If taking 39 bits as an example, this means the total addressable space will be 1 TiB of available addresses. The guest RAM is part of this addressable space, same as devices, PCI holes, ...
BAR (Base Address Register)
A base address register is part of the PCI configuration space of each PCI device, and it declares an extra memory region related to the device that can be found at this specific base address.