Very briefly since I'm headed out.
Standard acidosis is usually caused due to the imbalance of H+ and HCO3-. However HCO3- and Cl- are not the only major anions. When we measure the ionic balance with only Na+, K+, Cl- and HCO3- we come up with a difference, which is labelled the anionic gap.It is largely due to SO4 2- and negatively charged proteins. Any increase in these will cause an acidosis in which this "gap" is increased. E.g. lactic acidosis.