Units are needed to stick to a protocol so the interrupt-ask for (IRQ) traces may be shared. The PCI bus features four interrupt traces, INTA# by way of INTD#, all of which can be found to each unit. As many as eight PCI gadgets share the exact same IRQ line https://nathanlabsadvisory.com/iso-iec-19770-1-2017-it-asset-management/