Units are necessary to stick to a protocol so the interrupt-request (IRQ) traces might be shared. The PCI bus involves 4 interrupt traces, INTA# through INTD#, all of which can be found to every device. Around 8 PCI units share the same IRQ line (INTINA# by way of INTINH#) in https://nathanlabsadvisory.com/cloud-computing-security/