If a teen has set up money buckets, money will always come out of the Spending money bucket first. Teens can specify the order in which money is withdrawn from their other buckets (if they have created multiple buckets) once the Spending money bucket is depleted.
When auto-save is selected for a given bucket, that bucket will automatically fall to the bottom of the priority order list—meaning it will be the last bucket from which money is withdrawn, should all other buckets be depleted.
Teens can always reorder their buckets. Any new buckets created after they set their bucket priority order will be added after previously established buckets and before the auto-saved bucket. Auto-save can only be applied to one bucket at a time.