@anomalaus - tangent basis is "a surface-relative coordinate system for the per-pixel normals stored in the normal map, so lighting can be applied to the normal-mapped surface" - more detailed info on the Polycount Wiki here.

Tangent space means that the normal info determined by the tangent basis is stored relative to the underlying surface; in an object space map the normal info is stored relative to the object's orientation (& tangent basis is therefore irrelevant).

DirectX & OpenGL are both types of tangent space map but where the "required native direction" of the stored information is different - OpenGL is right-handed, DirectX is left-handed - again, the Polycount link gives more info under Swizzle Co-ordinates.

Poser has several renders - Superfly uses MikkT as the tangent basis but Firefly & Preview each use their own different tangent basis.

All that means that if a tangent space normal map is made using some random tangent basis it could result in lighting errors in all 3 of Poser's renderers.

If a tangent space map is made using MikkT as the tangent basis then Superfly will be fine but there could be lighting errors in Firefly & Preview.

Whether the tangent space map is OpenGL or DirectX only affects the direction & Poser can use either as the render in my previous post demonstrates.

The only thing I'm not sure of is if a map created as DirectX format on a Windows system would render with the correct directionality in macOS ;)

Now tell me, aren't normal maps fun!?!