A- This is the beginning of the graph, or what the sensor is seeing on the tape on the Left side of the range.

B- This is the middle of the graph, or what the sensor is seeing where the floor and the wall opposite the sensor meet. It is common to have a bit of a drop in the graph here, as the corner doesn't reflect as much light.

C- This is the end of the graph, or what the sensor is seeing on the tape at the Right side of the range.