Like many other vintage sewing machine enthusiasts, I've seen my fair share of machines on eBay listed as "Industrial Strength," "Heavy Duty," "Industrial Grade," "Light Industrial," etc. I never paid much attention to such claims, or what they actually meant, because I was searching with a purpose, already having a particular machine in mind before browsing listings. And, being a quilter, I'm more apt to sew quilting cotton (though I do occasionally hem jeans)--not exactly an industrial machine type of job.
Cut to a few days ago when I was trying to identify my new-to-me Singer 15-91.
The 99-13 is one of the few Singers that I also happen to own. At 3/4 size, it's not even a full size domestic, let alone industrial.
Long story short, this experience has opened my eyes. I can now see why so many vintage sewing machine collectors get frustrated with sellers who advertise domestic sewing machines as industrial grade. At the very least, it makes it more difficult to find accurate information, and at the very most, such sellers are taking advantage of inexperienced buyers and inflating prices.
There are several guides online discussing domestic vs. industrial machines, including one on eBay Guides, and on the Ultimate Sew & Vac site. The most informative I've seen by far, though, is on "The Vintage Singer Sewing Machine Blog". There you will see side-by-side photo comparisons of actual industrial machines and domestics, including their motors, and other parts. Thank you Rain! Your excellent photo comparisons really put things into perspective.