All addresses in a system, that are not marked as faulting, are assigned a memory type. There are two memory types in Armv8-A; Normal and Device. The memory type is a high-level description of how the processor should interact with the address region.
Note: In Armv6 and Armv7 there was a third memory type, Strongly Ordered. In Armv8, this maps to Device_nGnRnE.