Midpoint of the speed curve. The speed setting values are 1-255, irregardless of the number of speed steps you are using. I personally use 28speed steps; the controller goes from 1-28. However, internally to the decoder, 28=255 for the speed value. Controller value 14 is half of that, which is 128.
What the Mid/Max does is to set up what I'll call a pseudo speed curve. This uses only two setting values. If you set the MID value to, say 75, that means you get to the middle speed value faster (say Controller value 9 or so, vs 14); it won't be a straight line curve. If you set it above 128, then your midpoint will be higher, say a controller value of 20, let's say. This means overall, it'll take more increases to get to half speed.
The Max is a bit different; at max value, full voltage gets to the motor. Cutting the value back means your top speed won't be as fast, since you don't get the full voltage to the motor. I used this on one of my Walthers Rotary Snowplows. The motor that rotates the cutter wheel is only rated at 5-6 volts max (there's a current limiter soldered in line to limit it). So, when I installed a decoder to control the speed of the wheel, I set the MAX at basically under half value. This limited the current going to the motor as well as cutting down the speed to more realistic levels.
Kennedy