No source given
So, within a given hemoglobin molecule, there are multiple hemes. As per the positive cooperativity effect, binding of one or two of the hemes to CO increases binding affinity of the “other” hemes to O2. These “other” heme groups are the “heme groups not bound by CO”. I can see why it might be confusing but I can’t think of how to change the sentence to make it clear.