Someone can likely give you a more technical explanation - but to give an anecdotal example, my parents have German Shepherds which are grazers. They put a few cups of food in their bowl in the morning, and the dogs will eat throughout the day as they're hungry. They like treats, sure, but I wouldn't call them food motivated. My dogs are lab mixes, and if food is out they'll sniff it out and immediately scarf it down. When one was younger, if we left food on the counter or table while he was home alone, he would jump onto the counter and eat it. I would call my dog very food motivated.