Confirmation of acceptance for studies
Your education provider will send you a reference number called a confirmation of acceptance for studies (CAS) once they’ve offered you a place on a course. You’ll need to enter this on your visa application.
You must apply for your visa no more than 6 months after you receive the CAS.
the UK
You can only get a CAS for a new course if you’ve studied in the UK before and you’re:
studying for a new qualification at a higher academic level
studying for a new qualification that’s at the same level of study but relates to and adds to your previous qualification
re-sitting exams or repeating modules
applying for the first time to a new institution to complete a course you started somewhere else