FBX Advanced Settings - Normal Section

• OpenGL (Y+): Sets the normal map Y-axis to positive, which is the standard direction for OpenGL, Unity, Maya, and Blender pipelines. This dictates the interpretation of the green channel; if misconfigured, surface bumps will appear inverted or improperly lit.
• DirectX (Y-): Sets the normal map Y-axis to negative, which is the standard direction for DirectX, 3ds Max, and Unreal Engine pipelines. This is a common source of visual artifacts; a mismatch in channel direction can cause surface lighting to appear completely reversed.