Blender and Hitfilm normals problem

Hi!
I just made my first model and wanted to import it in Hitfilm 2 Ultimate.
Now the thing is, that some Textures (actually they are just materials from Blender) were not displayer properly.
I was able to solve that by flipping some normals, but I don’t understand WHY that solved my problem.
Maybe you can explain it to me.

I made a box and there’s this little plate, inside of the box, that isn’t displayed properly in Hitfilm.
In Blender everything’s fine.
So I flipped the normals, so that the it looked wrong in Blender but right in Hitfilm.
Why is that?

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