Bradley Kuhn writes about the phrase "open core". In summary, avoid that phrase and use clearer, unambiguous terms instead.

He also touches upon the practice of projects requiring copyright assignment before accepting contributions. Like Bradley, I dislike the practice.

If a corporation requires me to assign copyright to them, so that they have power to change the license, I need to be paid in a currency that helps me pay rent. The only reason they need the power to change the license is to make the code proprietary, for the purpose of extorting users for money. If they get money, I want money.

On the other hand, if I don't assign them copyright, I have a bit of power myself, and I can use that to keep them honest. If enough people do that, the corporation would have to rewrite all the code to make sure they have copyright on all of it, and can license it in a proprietary way.

There's another kind of copyright assignment, which the FSF uses: the FSF gets copyright, but the contract with them guarantees that they'll not violate the freedom of the code. In return, the contributor gets relieved from the burden of having to deal with license violators, and can rely on the FSF to do that instead. This is an acceptable bargain to me, even though I haven't contributed enough to GNU projects to do a copyright assignment.

There may be other entities than the FSF doing the same thing. I have not seen any others, though, so on the whole I consider copyright assignment as a way to trick me. With the exception of the FSF, copyright assignment smells dishonest to me.