The plys just refer to how many strands in the yarn. In the ply weight system, 2 ply would be lace, 3 ply heavy lace/lt fingering and 4 ply a fingering weight. A worsted weight is heavier than a sport weight.
The only references I can find call it a machine knitting yarn. Best thing to do would be to take some size 8s and knit it, see if it's the same as other worsteds. If it seems thin, then use some 6s and see how it compares to sport or fingering yarn.