Here's a million dollar question: What is the definition of 'clean'?
The USDA devotes 75 pages of 9 CFR to labeling policies and definitions, but five pages to sanitation? More specifically, 1/2 page to sanitary operations. So they have made it clear that they aren't going to give a clear definition.
The Food Code defines it as such:
4-601.11 Equipment, Food-Contact Surfaces, Nonfood-Contact Surfaces, and Utensils.*
(A) Equipment food-contact surfaces and utensils shall be clean to sight and touch.
(B) The food-contact surfaces of cooking equipment and pans shall be kept free of encrusted grease deposits and other soil accumulations.
© Nonfood-contact surfaces of equipment shall be kept free of an accumulation of dust, dirt, food residue, and other debris.
Now.... I understand how relative this topic may be, so I think we can already understand how and why regulatory bodies avoid defining this. However, just looking to flush out some helpful strategies to an organoleptic inspection process.