What is the symbol of a line in the middle of a rectangular box?

A line in the middle of the rectangular box of drainage legend represents drainage pipes or drainage facilities. According to the public information inquired, this line is sometimes called "drainage center line", indicating the center line position of drainage pipeline or facility. In the design of drainage system, the drainage center line is a very important parameter, which determines the position, slope and connection mode of drainage pipeline. The position and slope of the drainage center line must meet the design requirements of the drainage system to ensure the normal operation and drainage effect of the drainage system. In drainage legend, the drainage center line is usually represented by solid line and arrow, and the direction of arrow indicates the direction of drainage water flow.