| ACM Computing Classification System | ACM |
| AGROVOC | agrovoc |
| BIC standard subject categories | bicssc |
| Basic Subject Heading | bsh |
| DFG Classification | DFG |
| Dewey Decimal Classification | ddc |
| EU Data Theme | eu-data-theme |
| EuroVoc | EuroVoc |
| European Open Science Cloud | EOSC |
| Field of Application (NSF) | nsf:fieldOfApplication |
| Fields of Science and Technology classification | FOS |
| Göttingen Online Classification | gok |
| Horizon 2020 Topics | ec:h2020topics |
| Horizon Europe Topics | ec:hetopics |
| International Patent Classification | IPC |
| JEL Classification | jel |
| Library of Congress Classification | lcc |
| Library of Congress Subject Headings | lcsh |
| Mathematics Subject Classification | msc |
| Medical Subject Headings | mesh |
| Microsoft Academic Graph classification | MAG |
| National Diet Library Classification | ndlc |
| Nederlandse basisclassificatie | bk |
| Nippon Decimal Classification | ndc |
| OpenDOAR subjects | dnet:od_subjects |
| Optics Classification and Indexing Scheme | ocis |
| Physics and Astronomy Classification Scheme | pacs |
| Regensburger Verbundklassifikation | rvk |
| Sustainable Development Goals | SDG |
| UNKNOWN | UNKNOWN |
| Universal Decimal Classification | udc |
| Web of Science Subject Areas | wos |
| arXiv | arxiv |
| keyword | keyword |