This is pretty simple to me. This is a late 1970's era total-electric house. These are all over the place here. The top (MAINS) section contains no more than six disconnects (six two-pole breaker spaces). One of them feeds the middle sub-section via a 60 amp breaker and another feeds the bottom sub-section. The heavy loads, like heat, A/C, range or dryer circuits are placed in the top or "main" section. Being in Michigan, near the Canadian border?, I'm assuming that there is no central A/C. The breakers on the right of the top section would be (from the top-right), water heater, dryer and range.
The middle section contains 20 amp baseboard heating circuits, about 24 feet per circuit? The bottom section contains the SA and GPL circuits. Looks like Harry Homeowner might have stuck a few 120 V circuits into the middle section on the right side, but there's nothing wrong with that. I only say this since there appears to be a hardware store price tag (not the barcode label) on the top one.
No big deal in my book. Those panels, single or double sub-sections are fairly common around here. I don't necessarily like them, but I don't see a problem with them either. The design is good from a professional standpoint, but the safety factor for a homeowner is questionable.
