Maybe the documents that are not scientific but help your research in some other ways... I mean, sector data, experiencing of practices, law regulations, among others... It does not mean the information is bad or wrong, in fact is important also for research, just is a way to call this kind of information...
Grey literature are materials and research produced by organizations outside of the traditional commercial or academic publishing and distribution channels. Common grey literature publication types include reports, working papers, government documents, white papers and evaluations