Bits per Component, or (BPC) in PDF Optimizer refers to the color depth of an image, in the number of bits of information used to create each pixel of color component. With PDF Optimizer BPC refers to a step you can take to optimize images in a PDF document by reducing the resolution of an image from 16 bits per component to 8 bits per component. If a PDF document holds a lot of high-resolution images, this can make the file considerably smaller. This would be on top of reducing the size of images in a PDF document by manually setting a target resolution for each image. It is possible that an image in a PDF document would have a high resolution (1600 DPI perhaps) but also be set to 8 bits per component in color depth. So you could reduce the image resolution but do without the option to convert the bits per component from 16 to 8.