Module 

Header 
/Engine/Source/Runtime/Core/Public/Math/Vector.h 
Include 
#include "Math/Vector.h" 
See if two normal vectors are nearly parallel, meaning the angle between them is close to 0 degrees.
true if vectors are nearly parallel, false otherwise.
Parameter 
Description 

Normal1 
First normalized vector. 
Normal1 
Second normalized vector. 
ParallelCosineThreshold 
Normals are parallel if absolute value of dot product (cosine of angle between them) is greater than or equal to this. For example: cos(1.0 degrees). 