This file is the "publicized" version of the Assembly-CSharp.
You can create such a libary from any c# dll by using https://github.com/jacobEAdamson/publicize/releases/tag/v1.0.0
I didn't knew that myself, so it took me a while.
You can create such a libary from any c# dll by using https://github.com/jacobEAdamson/publicize/releases/tag/v1.0.0
I didn't knew that myself, so it took me a while.