In the first phase, we conducted an exploratory qualitative study to understand what code review comment usefulness means to developers. Further, with the help of the interviewees we identified signals suited to distinguish between useful and not useful code review comments. In the following subsections, we describe the study method and our findings. We deliberately focused on understanding comment usefulness from the perspective of the authors of source code changes because they are the people that actually make changes to the code based on feedback and they are the primary target for code review comments.