Thanks for the advise, but I do feel a little more baffled than before. I had no idea there were so many to consider. What has confused me most is the weight of the thread.
I am basically wanting some for machine embroidery, mainly FME on a normal sewing machine. It will be for things like couching, automatic stitch patterns and machine embroidery on water soluble fabric. I have a few rayon threads, and I find them a bit too shiny and quite like the duller more natural look you get with normal polyester thread. Would the cones of overlocker thread work for machine embroidery as they don't need to be that strong? They would seem to be a cheaper option but not sure if it would work well.