Units are needed to comply with a protocol so which the interrupt-request (IRQ) lines is often shared. The PCI bus involves 4 interrupt lines, INTA# by INTD#, all of which can be found to every device. Approximately 8 PCI devices share the same IRQ line (INTINA# by INTINH#) in APIC-enabled https://nathanlabsadvisory.com/iso-20000-1-it-services/