I assume you’re asking about job searching from an animator's perspective. I’d call the documents you send to employers a portfolio more than a demo reel in this case, but to give an example, for the Animator Grand Prix, we ask for 3 or more drawings that showcase one’s appeal as an animator 2 assignment drawings based on the following prompts: 1) A spine-chilling, terrifying experience, and 2) a fun family breakfast. All drawings, in our case, should also be on an A4-sized canvas. This is just 1 example, but I think including works with some kind of "theme" like those above are good as these works clearly showcase your abilities. It also helps to draw them as if you’re drawing a layout for key animation. Also, I think it’s essential to consider who the recruiter is and what kind of work they’d like to see.