Quantization is applied to DCT coefficients to decrease their precision. You may use quatization matrices to vary precision coefficients with different positions. Scale factor is applied to all coefficients in the macroblock in the same way. Quantization values are often inverted to increase performance. Scan matrices specify the way to reorder 8x8 quantized DCT coefficients to an array of 64, so that statistically bigger values are located earlier; the further the element is from the first AC coefficient in the scanning sequence (Figure "Default Scanning Sequence") the less important its value is for representing image and for further decoding.
The alternate scanning sequence is used for interlaced video and provides better compression for field-coded blocks:
