It went for above budget costs of running the operation. Per diem, travel expenses, overtime/night/Sunday premium pay, undercover costs, etc.
That being said, even before it came to light that ATF was dumping guns on the streets, their inspector general had already started investigating the use of these funds. I think one of the big offices spent $1.5 million in these funds and netted 6 arrests and 12 guns total. A complete waste of money.
Either a complete waste of money, or a way to pad the expenses to shuffle funds not actually spent elsewhere.