The tight bound tracks whether you're still constrained by the upper limit . If your first two digits match 's first two digits exactly, you're "tight" and can't exceed 's third digit. Example: . If you've placed , you're tight. Your next digit can only be -.
But if you placed , you're no longer tight. Your next digit can be -. Once you place a digit smaller than 's corresponding digit, you become "loose" and stay loose forever. You can never exceed anymore, so all future digits are free.