OK. I have done a bunch of research on this subject however I am not an expert.
Cattle grade grain is a rated as a II level. Human grade grain is III level. It specifically has to do with the amount of dirt and chafe in the end result AND if "they" put any poison in it to ward off rodents. If the package states no rodent deterrent (Or verbiage to that point) you just have to deal with a little bit extra dirt, stones and chafe. DO NOT TELL THE COMPANY (Like Agway) you are buying for human consumption or they will not sell to you.
Now if you buy seed grain there will (In almost every case) contain chemicals to prevent pilfering from rodents and a chemical to help jump start growth after it has been sown. STAY AWAY FROM at all costs if you are planning on grinding it in its purchase form.
Again, I am no expert.