If you are applying for a Tier 4 visa then you will require a Confirmation of Acceptance for Studies (CAS) before you can make a visa application. A CAS is an electronic document which is issued by the University but held by the Home Office on their Sponsor Management System and is identified by a 14 digit reference number. If you are an international student then we will normally issue you with a CAS once you have accepted an unconditional offer. When we issue your CAS we will email you to let you know your 14 digit CAS reference number and the information we entered on your CAS. You will need to know your CAS reference number and the information we entered on your CAS to apply for a Tier 4 visa.
You can find more information on CAS at the UK Council for International Student Affairs (UKCISA) website.