The UE4 engine's base functionality available in the editor is quite broad. The functionality from C++ code is actually grouped out into little sections called APIs. There is a separate API module for each important functionality in the UE4 codebase. This is done to keep the codebase highly organized and modular.
Tip
Using different APIs may require special linkage in your Build.cs
file! If you are getting build errors, be sure to check that the linkage with the correct APIs is there!
The complete API listing is located in the following documentation: https://docs.unrealengine.com/latest/INT/API/.