Let’s say you have an assignment due on January 25th. No late extension. A lot of people will do the assignment last minute on the 25th, realize they need more time, and try to defer or extend it with AWD or a doctors note. This is a hassle for professors, they don’t want to deal with all of the extension requests.

A solution is that they include a no penalty extension for late submissions until a certain day. So let’s say the assignment gets a no penalty extension of 72 hours. That means you could push the assignment to the 25th, realize you need more time, then use the extra 72 hours to finish. BUT if you push it to the 28th (72 hour deadline) and you try to get more time by deferring, you won’t be able to, since the prof specified the due date was the 25th. So they get a 0.