 So you don't know what flat inner means, not a problem. It is literally just the opposite of flat border. If you click on a flat surface, it'll apply the action to everything except the border. Doesn't care about poly groups, just the face angles. Here's what it looks like with bevel. Here's what it looks like with mask. Hope that helps, and as always, I hope you have a fantastic day, and I'll see you around.