It's very common to come across references in production protocols for images in the CCITT Group 4 tiff format. However not everyone that complies with these protocols necessarily understands what this format is.
CCITT stands for the Consultative Committee for International Telephony and Telegraphy. This organization developed the CCITT Group 4 format in the 1980s for use with fax machines. The same format may also be referred to as a Recommendation T.6 image. PDF images may also use CCITT Group 4 compression. CCITT Group 4 compressed images are lossless - the data integrity of the original is preserved - and can be compressed at a ratio of 15:1.
You can determine if an image is in the CCITT Group 4 format by right clicking on a file choosing Properties and clicking on the Details tab.
CCITT Group 4 compression is designed for bitonal images - black and white text images. The algorithm will not compress half tone images (those which use shades of grey) nearly as well. A continuous tone image would use a limitless number of shades of grey to depict an image. The commonly used methods of grayscaling uses either 16 or 256. Halftone replicates continuous tone through the use of tiny dots. See this example of how different an image can look using these three methods:
Lempel-Zif & Welch or LZW compression is preferred for grayscale tiff images. This method allows the grayscale or color images to be compressed at a ratio of 4:1. A CCITT Group 4 compressed halftone image may actually end up compressing at a size larger than the original file.
When dealing with document productions of many thousands of pages keeping files sizes to a minimum is a priority. This is why the CCITT Group 4 format for tiff images is so often used in the specs of document protocols.