In my opinion, fibers and lines are the same, being "fiber" the most common term. Both make reference to microplastics in elongated form. Although in some papers these microplastics could be called "fibers" and in others "lines", to my knowledge there is no research article (or other bibliographic source) making distinction between fibers and lines.

I agree with Pablo Reguera .

Line and (micro)fibers appear to refer to microplastics that arise from synthetic fibers. I am not aware of any publication providing technical differentiation between the two morphotypes.

Also, I strongly suggest the SCS system provided by Quinn and Crawford (2016) for MP classification.

Best of luck!