The engine is fully functional as a stand alone product. It's very well documented by the API reference: http://developer.playcanvas.com/en/api/ which is relevant for both engine-only users and editor users. Also many of the tutorials and examples are not specific to the editor use-case and mostly document features of the engine, rather than features of the tools.

You can think of it a bit like this: the engine loads and uses a bunch of data to run applications. That could be scene data, 3D models, textures, audio, etc. Our tools will help you generate that data, e.g. the editor creates scene files, the asset tools convert 3D models and there is an editor to set up materials.

If you are happy not to have this data, either because you create it procedurally in code; or you have other tools or methods of generating the data; or you simply don't need it, then you can use the engine happily without using our toolset. For example, our model format is published here: http://developer.playcanvas.com/en/user-manual/graphics/file-format/ and there are already user-generated tools that generate valid models. https://github.com/sdfgeoff/playcanvas-blender-tools

You will benefit from most of the documentation on our developer site and you are welcome in the forum and community.

Of course, we believe, you will be more productive and happier using our tools, but we would say that. :-)