YNAB offers three types of goals: Target Category Balance, Target Category Balance by Date, and Monthly Funding Goal. By default, YNAB constantly monitors and warns about the status of only the two latter types. The first type, Target Category Balance, never triggers a warning because it’s an open-ended goal (effectively, “I want to save $X but not according any schedule”).
The Warn When Target Balance is Not Reached setting triggers a warning any time the balance on a category falls below the targeted balance.
This is a great option when you want to maintain a funding level and add new money only when the balance dips below your goal.