Gap. There's a greater gap than usual (therefore anion gap acidosis). It's most helpful to consider the formula for the anion gap: Na - (Cl + HCO3). To protect the gap, any loss in HCO3 has to be made up by Cl or vice versa.
In non-anion gap acidosis, there is a drop in bicarb (acidosis) but a concomitant rise in Cl, making the net anion gap the same as normal serum. This is also called hyperchloremic acidosis. The best way to remember these is that they involve direct HCO3 loss or direct Cl gain.
In anion gap (aka high anion gap) acidosis, there is a change in unmeasured ions in the serum. Sometimes this is because there are extra anions (e.g. lactic acid) that acidify the blood, sometimes it's the direct addition of cations (e.g. iron) that also acidify the blood. Both of these drive the extra H+ to combine with HCO3, consuming HCO3 without a rise in Cl. There isn't a rise in Cl because the extra unaccounted for ions take care of the balance (which is another way of saying the HCO3 buffering system has done its job). A loss of HCO3 without a rise of Cl necessarily implies an increase anion gap.
It's kind of a confusing topic, but the central takeaway is that a direct gain or loss of Cl or HCO3 (respectively) has a non-gap anion acidosis.
