Where does the data in the Journal Subjects facet come from?
We get information on the journals that research articles belong to from CrossRef, which is a common metadata database run by the scholarly publishing community.
We enrich this data with journal subjects from Scopus, specifically the Scopus Subject areas described in this help document from Elsevier.
We use the Scopus subject areas because the mappings are freely available and cover a large proportion of the literature (unlike many science or discipline specific taxonomies).