CAS stands for Confirmation of Acceptance for Studies and is an electronic document that your institute/university issues when you accept the offer and fee is paid. The CAS is stored on the database that UK Home Office can see
The UK immigration rules require the university or institute to sponsor your application for a student visa (this is not the same as financial sponsorship). A CAS number confirms that we have made you an unconditional offer of a place to study with the institute, and that you have accepted this. You will need a CAS number to apply for a visa to allow you to study in the UK.
If you are going to study more than one course (for example an English language pre-sessional course followed by an academic degree course), you will normally need a separate CAS number for each course. In some circumstances, however, they may arrange a Combined CAS number for you covering both course