Barotrauma is not to be mistaken for free software or open source in 
a free-software activist understanding. Undertow Games retains the 
copyright to the source code and art assets and you are allowed to 
use them under the following conditions.

The source code, modifications or derivative works can be distributed 
only if they are intended to be used as a mod for Barotrauma. You are 
not allowed to create a standalone application, or use the code for 
commercial purposes or in your own projects. You are not allowed to 
remove or bypass Steam integration, SteamID authentication or any 
other DRM measures from the code.

By making a pull request on Barotrauma's git repository, you are 
stating that you're author of these changes or have rights to the 
changes you've made and agree to give Undertow Games permission to 
use, copy, modify, adapt, distribute, and publicly display that 
content, for free or as part of a commercial arrangement.

The origin of this software must not be misrepresented; you must not 
claim that you are the author of the original software. Derivative
works must be plainly marked as such, and must not be misrepresented 
as being the original software.

Undertow Games reserves the right to change this license at any time 
with or without notice, with immediate and/or retroactive effect.

If there's anything you don't understand or consider confusing, 
please email us your questions or suggestions: contact@undertowgames.com 

Tl;dr: the code is intended for developing mods for Barotrauma.