hadia mujahid is right. M3, also called sm+, is the final culturing media for the NPCs. Once the generation of the NPCs is completed, cells are kept in sm+ throughout the culture. You can find the media composition in Tab. 1 (basic media) and Tab. 2.
this is a media used for culturing the neural cells. it could be used for freezing, moreover you can look for the functions through the composition provided in this article.