The whole terminology of renal tubular acidosis can be confusing to the newcomer. Important features to grasp are:
The acidosis refers to the plasma, not the contents of the renal tubules. Indeed, in classical (type I) renal tubular acidosis the urine is often alkaline. Thus renal tubular acidosis is a metabolic acidosis of renal tubular origin.
There are three main types, named I, II and IV. Type III has become obsolete.
The three types have different sites of localisation in the renal tubule. However, again the terminology is confusing in that type II is proximal, type I distal.