Dimensions does not use Journal Classifications due to the difficulties and inaccuracies that these bring. For example, many broad-scope or multidisciplinary journals are almost impossible to logically categorize into a small number of relevant categories, and although many other journals may only have a small variation in subject between publications, an article level categorisation system will always be preferable if the categories are to be accurate.
Dimensions uses document-level categorisation based on automatic classification. This automatic classification is carried out based on noun phrase extraction and machine learning based emulations of standardised classification systems. All of this means that single documents can be rapidly classified, without compromising on accuracy.