The Units of Assessment (UoA) are the 34 categories of research used by the Research Excellence Framework (REF)2021 in the United Kingdom. Similar to the 2-digit FOR divisions found in Dimensions, the UoA categories span all of the sciences, humanities and social sciences but at a slightly more granular level than the top level FOR codes. For these reasons, the UoA categories provide an easy and accurate way of comparing or benchmarking Universities in the UK, and worldwide, across all of their research activities. Also, for Universities based in the UK, having the UoA in Dimensions enables an examination of research activity in these categories over time, instead of just a snapshot every 7 years, as is provided by the REF process itself.

As with all of the categorisation systems in Dimensions, the UoA system is an algorithm-based model of the Units of Assessment, created through machine learning on pre-labeled documents. The UoA categorisation system in Dimensions is not a record-for-record copy of the UoA classifications, but a very accurate emulation. This means that there may be a small number of cases in which a publication submitted to the REF process was defined as being in one category, but when found in Dimensions it is labelled in a different category, or in more than one category.