NeoEngine isn't a complete game engine ready for you to start moding, instead it's a complete set of libraries that you can use to create such an engine. To use an analogy, it's not a piece of prefabricated furniture that only need a paintjob, it come in pieces that you have to assembly yourself in the way you like.
NeoEngine is built with platform and API independency as the main goal. This means that if you build your game using the NeoEngine abstractions for services such as rendering, audio, file/resource management and networking, your game will run on all NeoEngine supported platforms with no changes to your game code.

NeoEngine is also designed with an "open" API in mind. This means you can easily change, remove or add components you wish. Maybe the most obvious example of this would be the space paritioning algorithms. The engine natively deals with ABT, BSP and quadtrees, but if you have other needs it is a simple task of integrating your own solution in the engine by deriving from the base abstract interfaces all space partition algorithms build on.

NeoEngine is Open Source, released under the Mozilla Public License. This means you can build proprietary, closed source games using the engine, as well as Open Source games (of course). If you make changes in the core engine, that is the code you got when you downloaded NeoEngine, you must release your changes to the public through the MPL license.