[libvirt-users] can someone explain why the domain, function bus, slot function is used in this xml

I don't understand the domain bus slot fuction in below xml,becase, this is used when deal with management of host devices that can be handed to guests via passthrough as <hostdev> elements in the domain XML.( from http://libvirt.org/formatnode.html) I don't see it use passthrough feature, why it has "domain" bus" slot" and function here <disk type='file' device='disk'> <driver name='qemu' type='raw'/> <source file='/kvm/images/disk/vmname_boot.img'/> <target dev='vda' bus='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/> </disk> <interface type='bridge'> <mac address='52:54:00:07:ab:b0'/> <source bridge='br0'/> <model type='virtio'/> <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/> </interface> which originally comes from this blog: http://blog.allanglesit.com/2011/03/kvm-guests-manipulating-libvirt-xml-f... Any point?