It concerns some knowledage about color :
first, any color can be measured with "Gray level", for example WHITE=0 BLACK=255
1 multi-recognition, means the cameral captured the picture using 256 gray level (any one between 0~255)
2 Binary recognition, means the cameral captured the picture using 2 gray level (0 or 255)
if the Mark of PCB condition is ok, use Multi-recognition, the accuracy is better.