Surface Relief pertains to elements, both recessed and extruded, that you wish to show on your model at your chosen scale.
Why is it relevant? Knowing your chosen printer's tolerances for surface relief is necessary because you may need to Accentuate or over-accentuate particular details in order to have them show up as clearly as you would like. Of course this is determined by knowing the scale you will be printing at and the machine you will be printing on. If you are unsure of either variable, you can at least draft with the concept in mind.
An Example. If I want to print a surface with a physical texture map, scales or tiles or rocks or hair or even lettering, and I want the effect to show a crisp relief on the surface, I would take the minimum suggested value from the calculator and apply this to my model. So, a model printed at 1/16th scale (also known as 1"=16' or 1:192) and I want to print on a FDM printer, the calculator will show that the Surface Relief detail minimum tolerance is .02" and I would need to make sure that the depth of my surface texture is at least 3.8"
In a similar circumstance, if I wanted to print at 1:10 on a FDM, the same relief detail would only require a depth of .2" and a metric equivalent would simply be off by a factor of 25.4.
How much do you want your texture to show?
The blocks on the left were very subdued upon the customer’s request using half the CADspan Calculator's suggested minimum value while the brick work and banding on the right used the full minimum value. The fuzziness on the surface is an attribute of 3D printing with the ZCorp machines, not a texture.