backward-forward counter

例句：

1. 1. The backward-forward counter is used to measure the distance traveled in a specific direction.
   倒退计数器用于测量在特定方向上行进的距离。
2. 2.The forward counter is used to record the number of items sold during a transaction.
   向前计数器用于记录交易中售出的商品数量。
3. 3.The retrocounter is used to calculate the total amount spent on an item after it has been returned or exchanged.
   倒退计数器用于计算商品退货或交换后的总花费。