Products are required to observe a protocol so that the interrupt-request (IRQ) strains could be shared. The PCI bus consists of 4 interrupt strains, INTA# by means of INTD#, all of which are available to every unit. Up to eight PCI products share the same IRQ line (INTINA# via INTINH#) https://nathanlabsadvisory.com/testing-for-compliance/