I agree that grading can be subjective, and that it's best to provide good pictures of the signs that you are trying to sell. Then let the potential buyers make their own assessments regarding condition.
But if you're purchasing signs for investment purposes, or with plans to resell, you really should have at least a basic understanding of the grading system and how it affects value. Here's a link that describes sign grading, using illustrated examples of a porcelain coke sign at various grades/conditions. I've found that it's pretty consistent with the grading used by the more well-known auction companies that specialize in antique advertising.
https://www.earlycoke.com/porcelain-enamel-signs-condition
