What is the "TangentsW" attribute inside the FBX file format for?

Question

sometimes it is -1, sometimes is +1. Do I need it for the rendering of the normal maps?

Answer 1

Looks like the tangent handedness that's needed for proper normal mapping. It can be used for calculating the bitangent for TBN matrix. Example in GLSL: vec3 bitangent = normalize( normalMatrix * (cross( aNormal, aTangent.xyz ) * aTangent.w) );