Chris Gagnon, a Lead Programmer at Epic Games, goes into detail on what subsystems are, how to use them, their benefits, and other considerations to keep in mind.
Topics
Rewinding a networked game
Implementing Instant Replays (Killcam)
The Unreal Engine Game Framework: From init main() to BeginPlay
Unreal Engine Network Compendium
How to make tools in Unreal Engine
Making UIs With C++ in Unreal Engine
All about Soft and Weak pointers
GASShooter