<p>The AB subscore of the BC test is used by some universities for placement purposes, in that they allow you to use either your BC score or your AB subscore (as if you got that score on the AB test).</p>
<p>For example, suppose you get a 2 on the BC test, but an AB subscore of 5 (not sure if this is possible, but use this as an example). Some universities may allow you to place into math courses as if you scored a 5 on the AB test, even though a 2 on the BC test otherwise gives you no placement into more advanced math courses.</p>