Hi @LearningSlicerYay ,
Relative Anisotropy is defined as the ratio of the anisotropic part of the diffusion tensor to the isotropic part (Basser, 1994). The implementation is here: https://github.com/zhangfanmark/Slicer/blob/69c4b90c62ab400f9f7862bdbfd96fc02998f45d/Libs/vtkTeem/vtkDiffusionTensorMathematics.cxx#L1006 Different to FA, the denominator is trace, instead of norm.
Determinant is just the determinant of the tensor matrix. You can find the math definition here: https://www.chilimath.com/lessons/advanced-algebra/determinant-3x3-matrix/
Regarding Num_Clamp_Excluded, there are some computed values out of the expected range for some reason. E.g, FA value may be negative due to noisy signal values. These values are excluded when computing the tract statistics. Num_Clamp_Excluded is used to record the number of such voxels.
Regarding NAN, this means not-a-number, and it should happen when the input tract vtk file is empty, i.e. not fibers.