Since all 3D stress/strains are tensor quantities (even for purely linear models), MEs project them into principal axes so that you can see their angle and their peak tesnsile/shear stress.

https://en.wikipedia.org/wiki/Cauchy_stress_tensor

Look at Mohr's circle:

https://en.wikipedia.org/wiki/Mohr%27s_circle

Within an object the stresses and strains (and principal components) vary as you move through it. That's part of what makes predicting failures and effects of thermal expansion so complex, etc difficult even ignoring nonlinear effects and failure modes (e.g. crack propagation).