Understanding the parts of the CvType

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1) What is a color channel?

2) What do all the parts of a CvType represent, e.g. CV_8UC3. Like what does 8, U, C, and 3 mean

3) in the public static class CvType what does this syntax mean with the (3)

public readonly static int CV_8UC3 = CV_8UC (3);

here's a reference to the beginning of the public static class CvType if you need it public static class CvType {

    // type depth constants
    public const int CV_8U = 0;
    public const int CV_8S = 1;
    public const int CV_16U = 2;
    public const int CV_16S = 3;
    public const int CV_32S = 4;
    public const int CV_32F = 5;
    public const int CV_64F = 6;
    public const int CV_USRTYPE1 = 7;

    // predefined type constants
    public readonly static int CV_8UC1 = CV_8UC (1);
    public readonly static int CV_8UC2 = CV_8UC (2);
    public readonly static int CV_8UC3 = CV_8UC (3);
    public readonly static int CV_8UC4 = CV_8UC (4);
    public readonly static int CV_8SC1 = CV_8SC (1);
    public readonly static int CV_8SC2 = CV_8SC (2);
    public readonly static int CV_8SC3 = CV_8SC (3);
    public readonly static int CV_8SC4 = CV_8SC (4);
    public readonly static int CV_16UC1 = CV_16UC (1);
    public readonly static int CV_16UC2 = CV_16UC (2);
    public readonly static int CV_16UC3 = CV_16UC (3);
    public readonly static int CV_16UC4 = CV_16UC (4);
    public readonly static int CV_16SC1 = CV_16SC (1);
    public readonly static int CV_16SC2 = CV_16SC (2);
    public readonly static int CV_16SC3 = CV_16SC (3);
    public readonly static int CV_16SC4 = CV_16SC (4);
    public readonly static int CV_32SC1 = CV_32SC (1);
    public readonly static int CV_32SC2 = CV_32SC (2);
    public readonly static int CV_32SC3 = CV_32SC (3);
    public readonly static int CV_32SC4 = CV_32SC (4);
    public readonly static int CV_32FC1 = CV_32FC (1);
    public readonly static int CV_32FC2 = CV_32FC (2);
    public readonly static int CV_32FC3 = CV_32FC (3);
    public readonly static int CV_32FC4 = CV_32FC (4);
    public readonly static int CV_64FC1 = CV_64FC (1);
    public readonly static int CV_64FC2 = CV_64FC (2);
    public readonly static int CV_64FC3 = CV_64FC (3);
    public readonly static int CV_64FC4 = CV_64FC (4);