Which harness slots should I use for rear-facing versus forward-facing?

For rear-facing use, harness straps must be at or below the child's shoulder height, using one of the lower harness slot positions. For forward-facing use, harness straps must be at or above the child's shoulder height, using one of the three upper harness slot positions. Never use the highest harness slots rear-facing, and never use the lowest harness slots forward-facing, as the harness will not properly restrain the child in a crash. (Page 38)