Machine Stress Rated Lumber (MSR)
Lumber is graded in two ways, visually by qualified graders who inspect the lumber for any visual defects or damage and by machines that apply forces on the lumber to test the bending strength and for stiffness.
The MSR grades of lumber have been listed in NSD (National Design Specifications for Wood Construction) for many decades and are designed numerically by their Fb (design bending stress) and E (modulus of elasticity or stiffness) values.
Thus the lumber grade 1650f-1.5E designates a bending stress of 1650 psi and a stiffness of 1.5 million psi.
2100f-1.8E lumber designates a bending stress of 2100 psi and a stiffness of 1.8 million psi.
Other design properties are listed in the NDS a publication of the American Forest and Paper Association (AFPA) providing an appendix of lumber species and allowable stress factors.