I have one spool of thread that is labeled as tex 40. It's definitely heavier and thicker then other thread i've used. I used it on one quilt and you can definitely see the difference if you are looking. I plan on using it on a quilt that has a lot of denim and heavier weight fabrics. It doesn't disappear into the fabric, not even sandwiched flannel. However, this was done on a singer 31-19. I wouldn't trust my brother to play nice with a heavier thread.
I see what you mean about there not being a standard. The tex 40 is military black. It says GOVT Cotton Wrap Core. I also have a bunch of other military thread but none of those have a tex #. Instead some list sizes of letters like A and E and others have numbers like 3.
It wouldn't surprise me if there is a different sizing system based on the purpose of the thread. Some of my threads were intended for flight jackets, parachutes, duffel bags, etc.