You should have a writing sample of 8-10 pages, double-spaced, available as you prepare for interviews. DO NOT upload this to CASE; you will only need it at the interview. Employers will sometimes ask for a writing sample before interviews begin, but generally do not ask until the actual interview. It is essential to have your writing sample with you at your interviews.
The short answer is your best writing, whatever you think accurately and most positively reflects your legal reasoning and analytical skills. Some students use writing from a summer project, but before you do you need to first confirm with your supervisor and be sure to redact any confidential or client-identifying information. Most students use a portion of their legal research and writing brief. It is okay to use a part or section of a longer writing, because your writing sample should be between 8 and 10 pages, but no more than 12 pages max. If you use a section of a longer writing, you may include a cover page with your name, and a brief (no more than a short paragraph) description of the facts, procedural posture, your position, or any other relevant facts. Don’t get caught up in restating the statement of facts, just give the reader an overview of what they will be reading and an idea of where it fits in the longer writing sample. Be sure to print it out and double and triple check it for punctuation, grammar, and stylistic errors. Be prepared to discuss your writing sample in an interview, whether it is an OGI or a callback.