I think he means that there is only 447114 permutations of possible balances (at maximum). I assume this is so you can assign a certain index to a particular balance, to save space? I don't know 
I am guessing so too - although I think it would make things rather complicated (as you would have to keep a "count" of how many accounts have that balance and then "remove" that balance when that count goes to zero, etc.).
To my thinking a simple B+Tree keyed on "account id" is much easier to picture and to work with (even if it takes a lot more storage).
Guess a trade-off between simplicity and size will have to be made in order to come up with an "optimal" solution.