I was cruising around a local cruise night and happened to look at my heater control plate. Under "heating" it has "HTR", "DEF", and "DEICE". I hardly ever use the heater because it's a summer cruiser. But the DEF and DEICE got me wondering what the difference is. I am assuming the heater one blows out the dash vents and possibly the defroster. Defrost and deice sound the same to me. They both sound like they should blow out the dash. What is the difference is the two settings? I know I could set the fan on hi and see where it blows out, but who knows if it's operating 100% correctly anyways. It's a 1970 Monte Carlo if it makes a difference. HTR is on left and DEICE is on right for the locations. It also has AC, but doesn't work currently.

Defog may have air only, no heat.

From left to right it has OFF, (next 3 are under "air conditioning") MAX, NORM, and a diamond shape, then VENT (not under heat or AC), (next 3 are under HEATING), HTR, DEF, DEICE. If DEF is defog with no heat, wouldn't it be in between the AC and Heater sections?

My guess would be the deice would involve the AC to run on the heat cycle. Part of the AC function is to remove the humidity from the air. Running the heat through the AC evaporator would hit the windshield with warm, very dry air, and would probably remove the ice faster.
Most of the new stuff automatically runs the defrost through the AC evaporator.

This.

Most vehicles with AC will turn on the compressor during defrost settings. Been that way for decades.

Fun fact, government regulations on how quickly the windshield can be cleared are a big reason you can't buy a car without AC anymore.

my 87 Diplomat had that, but if you pulled the pushbutton out you'd get defrost without AC. I can;t remember if my 72 Dart had that, but my 65 Cuda did not, till i retrofitted a later control module in and now it does.