SplineMesh CalcSliceTransform Function in Shader

Development Programming & Scripting C++
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1

I was learning SplineMesh and I read the shader code in localvertexfactory.ush. I was confused at two line, here is the code(in function CalcSliceTransform):

// Build overall transform
		float3x3 SliceTransform3 = mul(transpose(float3x3(SplineMeshDir, SplineMeshX, SplineMeshY)), float3x3(float3(0,0,0), XVec, YVec));
		float4x3 SliceTransform = float4x3(SliceTransform3[0], SliceTransform3[1], SliceTransform3[2], SplinePos);

My question is that why SplineDir is not used here while I found some similar code in CalcSliceTransformAtSplineOffset in SplineMeshComponent.cpp and it used the SplineDir variable

2

I want to know why the first row set to float3(0.0, 0.0, 0.0) instead of SplineDir. Any answers will be helpful.