There are 67 Units of Assessment Higher Education Institutions can submit work to, though in practice, most submit to far fewer. At Aston for example, we submitted four Units of Assessment (Business Studies, General Engineering, European Studies and Subjects Allied to Medicine).
The work in each Unit is assessed by an expert panel, made up of senior subject-specialist academics. The panels score submissions based on three criteria, each of which are weighted, with outputs being the most important:
Outputs – published research, journal articles, conference papers, patents
Esteem – other notable achievements that may have brought prestige, such as hosting conferences, editing journals etc
Environment – institutional performance, research income, business collaborations
Using these scores they then produce a quality profile, which is what is used to rank institutional performance.