Units are needed to adhere to a protocol so the interrupt-ask for (IRQ) strains can be shared. The PCI bus incorporates 4 interrupt strains, INTA# through INTD#, all of which can be found to each product. Up to 8 PCI gadgets share the exact same IRQ line (INTINA# through INTINH#) https://nathanlabsadvisory.com/nesa-compliance/