I believe the intent is that you can use anything you have bought or made in the past, as long as it wasn't made in preparation for the game jam. So say you made a tree asset for a previous project? You could use that for one now. You know this next game jame is going to be about Power Surge(maybe), so you pre-build a generator asset? That's against the rules.
Code would be the same IMO, as long as it was originally created for another purpose, and is re-usable, it wouldn't break the fair to use rule. Obviously, the more you have from older projects, the easier it gets to re-use old code, but honestly I think thats sort of the point. You get to build a library of code and assets from previous projects by doing these weekly, and then can re-use or refactor those old assets to work for newer projects as part of the learning process.