From what I've read in the 88F6281 documents on this page
, the interrupt controller can assert two TYPES of interrupts: either IRQ or FIQ. The mask bits of each type define which type is generated on each interrupt cause. That is, based on the FIQ low/high mask settings, a FIQ can be caused by any interrupt source, internal or external.
I don't know if this helps and, of course, I could be completely wrong.