Congress directly controls BA and through it the level of spending agencies can undertake. BA is the usual measure when discussing appropriations bills.
By contrast, for programs that have permanent authority to spend money, for example for Medicare or Social Security, the usual measure to discuss is the actual rate at which these programs spend money, i.e. outlays.
When comparing total spending (annual and mandatory) outlays are generally used. And it is the difference between outlays and revenues that determines budget deficits.