Silk thread is best for buttonholes, pad stitching, and hems. Don't put topstitching thread in the bobbin because its thickness can ruin the tension mechanism. Use the same thread type in both the upper mechanism and the bobbin. Thread manufacturers have been known to drop their quality once their name was established. Stores will always continue to sell junk thread because many consumers do not take the time to learn more about thread quality.
Bad thread is often the sole source of many sew-ers problems.
I enjoyed reviewing these 5 pages of Hazen's book.