BAL stands for "balance", and FSV stands for "financial state vector". Let's say that you have a transaction T, where $500.00 is debited to one account and credited to another account; in mathematical notation, this can be written as:

  T = I + |A>$500 - |B>$500

or:

  T = I + |A>$500 + |B>$500

depending on what basis you are working with (since you can just define |B> to point in the opposite direction if you want a different basis vector).

And then, you can have:

  <BAL|T|FSV> = 0

It is easy to see that this equation is valid.