Sounds like local lingo to me more than anything. There may be some oats that can overwinter in the south, but up north, I think all oats are the same with some minor differences depending on objectives and farming challenges. Things like
Days to maturity
Stalk height
Test weight/protein %/groat (don't know what groat is)
Lodging resistance
Disease resistance
Leaf blade width (wider more geared towards forage)
When you plant in the later parts of summer or early fall, those maturities matter a whole lot more. As heat units per day dwindle, a shorter duration oat may still head out, where a forage oat may only be ten days behind a short maturity oat in July, it could be many weeks later to head in the fall, making it much easier to keep as forage instead of turning to straw.