Say you have a sensor that detects an item made of two parts--one on top of the other, which elevates it's total height. Now there's a sensor that detect the presence of the bottom "Part", and a second sensor that detects the upper part "Elevation".
Place the parts on a moving conveyor. When the bottom sensor detects, the upper sensor should detect as well. When this occurs, load a "0" into your BSL word address. When only the bottom sensor is triggered, load a "1" into the word. "Downstream" in the BSL the "1" will trigger an alarm or reject sensor to kick the incomplete part off the line.
Just one of many apps for a BS...