The Academic Technology Approval Scheme (ATAS) is run by the UK government's Foreign and Commonwealth Office (FCO). Under the scheme, International students who are applying to study for a Masters-level qualification or higher (including undergraduate Masters degrees) in certain sensitive subject areas have to apply for government clearance before they can study in the UK. Clearance is granted in the form of an electronic ATAS certificate, issued by email.
More information is available on the University's ATAS webpages: sheffield.ac.uk/admissions/atas
You can find out if you need ATAS clearance on the Foreign and Commonwealth Office's webpages: gov.uk/guidance/find-out-if-you-require-an-atas-certificate