“…The use of UAT subject headings in this classification scheme allows for scientific accuracy within subject areas and narrower and broader searching. As of 2015, the UAT had 1,906 terms, a range of 12 hierarchical levels and 15 top level concepts (Frey et al 2015). On the release of UAT v.1, the scheme was updated to include 1,834 terms, a range of 10 hierarchical levels and 11 top level concepts (Unified Astronomy Thesaurus 2017a).…”