I thought Signature was a serger thread...? I could be wrong. I had some a few years ago and threw it out because it broke so easily.
I agree with previous posters who said to try the break test, where you tug it hard to see how easily it breaks. I was hired to finish a quilt that a friend had started, and she just didn't have the patience to finish sewing the binding down. She was using some kind of thick, heavy, cotton thread. I was able to snap it with one little tug. I use a double strand of Bottom Line. Despite the fact that it is a fine 60 wt thread, it is very strong, and I can't get it to break at all.