It’s the core code for the game. In theory someone could use this to mod the game, or even expanded the game themselves.

As someone commented on the page, this is probably some code shared to third parties to include DRM or be sold on a store. It’s incomplete and doesn’t compile. It could be used to develop mods to an extensive degree, but I’m not sure how much of it would be legal as those tools would have been written using some leaked pieces of software which belong to Ubisoft. It can supposedly not be used to guide an open-source reimplementation of the game, like what was developed for DOOM, the first Diablo game, Moerowind, etc, as per the legal terms, those reimplementations must be developed from blank page, without looking at the original source code.

Butt yeah, I think this can be useful to develop mods and tools.