Your Safe Dollars is the minimum amount of money you want to have in your Spending Balance before using AutoSave.
If your Spending Balance is less than your Safe Dollars amount, AutoSave will be paused. If your Spending Balance is more than your Safe Dollars amount, AutoSave will be active.
If your Spending Balance is more than your Safe Dollars amount, but AutoSave would cause your Spending Balance to be less than your Safe Dollars amount, your AutoSave will be partially completed up to your Safe Dollars amount.