Andrew Morton submitted some documentation explaining the use of the "Signed-off-by" and "Acked-by" tags added when patches are submitted for conclusion into the Linux kernel. "The Signed-off-by: tag implies that the signer was involved in the development of the patch, or that he/she was in the patch's delivery path," the documentation explains, "if a person was not directly involved in the preparation or handling of a patch but wishes to signify and record their approval of it then they can arrange to have an Acked-by: line added to the patch's changelog." When asked about the possibility of including "Tested-by" tags, Andrew replied, "I think it's very useful information to have. For a start, it tells you who has the hardware and knows how to build a kernel. So if you're making a change to a driver and want it tested, you can troll the file's changelog looking for people who might be able to help."
The thread went on to discuss if Ack and Nack patches were useful from non-maintainers. Andrew suggested that a without additional information they don't offer much, "it's better to just provide constructive, detailed technical comments and from that it becomes pretty obvious to all parties whether or not the patch has a future. If you did properly provide that useful feedback then the 'ack' or 'nack' bit becomes redundant." He went on to stress the need for useful feedback, "frankly, I don't trust a simple 'ack' much at all. It's the kernel equivalent of 'whoa, kewl!'"