Well, absolute speed commands are just that, absolute. If you see a 16 in the speed setting for TMCC or Legacy mode, a given engine will have the same speed command and, given the same environment, should run at the same speed.
Relative commands are just an increment or decrement to the current speed step that the engine is running at. There is no sense of an absolute speed setting.
Try this and you'll see the difference.
For TMCC or Legacy settings in the remote, with a compatible locomotive, run the speed up to a mid-range value. As the engine is running, turn off the power. Turn the power back on and just move the throttle one step. The engine will take off at the set speed again.
Now, using the CAB1 or R100 modes, do the same test. When you turn the power back on and go one click on the throttle, the engine will be at dead slow, as it's taking one speed step relative to the current setting, which is zero.
