Just found this in the engine and not entirely sure what it does:
.h
private:
...
/** A list of all available SoundNode classes */
static TArray<UClass*> SoundNodeClasses;
.cpp
TArray<UClass*> USoundCueGraphSchema::SoundNodeClasses;
This line in the cpp is outside of a function. Is that so the static member can be used inside a none-static function or something?