Hello Eddie,

Tensile stress is positive and compressive stress is negative.

There are three Principal Stresses: Maximum, Minimum and Middle. 

Plot the Minimum Principal Stress to see the extreme value of compressive stress, which is the minimum value of the Minimum Principal stress.

The maximum value of Maximum Principal Stress is good for brittle materials to compare with Ultimate Tensile Strength.  The minimum value of Minimum Principal Stress is compared with Ultimate Compressive Strength for brittle materials.

The maximum value of Equivalent Von Mises Stress is used on ductile materials to compare with the Yield Strength.

You also need to learn about stress singularities. You will find that the extreme values of stress will change with element size at that sharp corner you show in the image.  If you mesh with elements twice as large, the stress will go down a lot. If you mesh with elements half as large, the stress will go up a lot. If you continue to make the element size smaller and smaller, the stress will keep increasing without any limit (until your computer runs out of resources). The way to find the true stress is to open the geometry and add a blend radius that is needed on that edge. Brittle materials can't have sharp corners becuase that is where a crack will start.  If you have a ductile material, you can add a plasticity material model and allow the elements at that sharp corner to go past yield.