PCI slots are not all the same. The PCI protocol (or "PCI Spec", as it is usually called) orders the slots from lowest to highest, using alpha-numeric denomination. The term "alpha-numeric" simply means that the slots may be labled using a letter, a number, or both (for instance, Slots A, B, C ; Slots 0, 1 ,2 ; or Slots A1, A2, A3 ). Digidesign uses mainly numbers to refer to slot order.