Is the "compute tangent space per fragment" ignored when bringing in tangents in FBX?

I know that compute tangent space per fragment is an option in mikktspace. I know that compute tangent space per fragment is an option in mikktspace, and I understand that they need to be used differently for different engines like unity and unreal.

What I'm wondering is why I need the option if I'm bringing my own tangents calculated in maya into Substance. If I use custom tangents in the game engine, should this option be checked? Or will this option, which is mikkt-specific, be ignored for custom tangents?

Thank you.