I use them to mark cutting lines, sewing lines, placement lines, etc. Any writing ink pen the manufacturer will say not made for fabric, the company does not want to be liable for any damage. I have a dishtowel my grand drew an outline of her hand using a Frixon pen. I sewed around it an it's been used and washed many times over the years. The fabric is fine so I don't think it will harm fabric over time.

I love these pens because of the fine point. I use chalk seldom, as the one I can see best is yellow and sometimes I can't get it 'out'. I found Frixion refills online at a fraction of replacing the whole pen. I only use blue and black, as the lighter ones can sometimes not disappear.