Yes, the terms refer to the throat length. It's not cast in concrete, but a long arm is usually 18" or longer, a mid arm is 9 to 17 (I've also heard 11 - 17) and anything under 9 (or 11
) is a domestic machine. Normally long arms are on frames, mid arms and domesics can be on a frame or sit down. I would call yours a mid-arm sit down. I would also call it a wonderful set up!