Found this information on Superior Threads website.
Glazed. Glazing involves heating the thread and then coating it with waxes, starches, and other chemicals. Glazing results in a glossy thread with a hard finish. Glazed thread is stiffer than unglazed thread and has a wire-like look and feel. Most professionals do not recommend glazed threads for machine work as the glaze rubs off and gums up the machine. Although usually not labeled as such, glazed cottons are recommended for hand quilting only.
You might find the website informative about a lot of thread questions...they have an education section. Here is the link.
http://www.superiorthreads.com/educa...bel-terms-mean