The Address bus is used to access memory locations on high-speed devices. It is connected to a CPU or GPU, and splits the address space into four slots, which may overlap. Each slot has an offset and a size - the offset is where in the address space that slot begins, and the size is the size of that slot.
For example, if you have an address bus with slot 1 set to an offset of 2000, a size of 5 and a data port connected, then addresses 2000-2004 of the address bus will be forwarded to addresses 0-4 of the data port. Addresses 2005-2007 will not be forwarded, because the size is 5 - so addresses 5-7 of the data port will be inaccessible.