Hi Josiane,
There’s a big difference between the operation of a .tsf vs. .txt file here. The former allows you to attribute some value per streamline vertex, whereas with the latter you can only attribute a value per streamline. If you have a statistical model parameter that is defined per fixel, the former simply involves sampling at each streamline vertex the value stored within the underlying fixel (currently via fixel2tsf). The latter would necessitate at least one extra operation.
What you’re most likely looking for (the former) is described in this Wiki page.
Rob