In conjunction with search terms, or instead of them, different classification systems and codes can also be used to describe the contents of a document. Classification codes are often hierarchical and independent of language. Databases often use their own classification systems and therefore classification codes and their use should always be checked from the instructions of the database being used. When looking for classification codes, one can also find helpful the classification codes of relevant references found earlier.
For example, the Abi/Inform database uses its own classification, in which the code 9520 refers to a small business. The code can be used as a search term. Image source: ProQuest (ABI/Inform) <http://search.proquest.com> 3.9.2014
For instance, the following classification systems are used in Oula:
The UDC classification is used in many research libraries. For example, the books can be arranged on the shelves by subject using the UDC classification. Image source: UDC MAster reference file <http://trip.cc.tut.fi/udc>