Advanced Decoder have speed table which allow one to customize the actual physical speed of the locomotive to a given speed step. In essence you create a speed step vs speed curve. THere are multiple purposes for using speed tables.
1) relative speed matching locomotives to each other.
2) Speed matching to a prototypical scale speed.
3) Adjusting a Yard locomotive to have as speed curve that facilitates quick engine switching response a limited throttle change. Great for one handed operation.
Dave Heap has done a lot of investigation in the growing trend of DCC decoder manufactures moving away from the NMRA DCC standards. In this discussion, it is in regard to how speed tables are supposed to work. Each decoder manufacture believes they have a better way to do a speed table than the NMRA. But it only creates more confusion in the end since there is no standard.
Here is Dave's Findings as posted on an email on 10/6/14.
The NMRA standard specifies that when:
1) bit 4 of CV29 is 0, the speed table is set by CVs 2, 5 & 6 only. The values of CV66 to CV95 are disabled.
2) bit 4 of CV29 is 1, the speed table is set by CVs 66 to 95 only. The values of CV2, 5, and 6 are disabled.
Tsunami decoders:
If the speed table CVs 67 to 94 is active, the value in CV 2 is still effective and is added to ALL points in the curve, effectively shifting the whole speed table up.
QSI Decoders:
If the speed table CVs 67 to 94 is active and the values in CVs 2 & 5 are non-zero the whole speed table is rescaled to fit between the values in CVs 2 & 5, while preserving the curve shape. (If you have already set the curve to be compressed, it is expanded to fit between CVs 2 & 5).
ESU Decoders (V4 or Select):
If the speed table CVs 67 to 94 is active, and the values in CVs 2 & 5 are always active and the whole speed table is rescaled to fit between the values in CVs 2 & 5, while preserving the curve shape. But unlike the QSI decoders, CV67 is fixed at 1 and CV94 is fixed at 255 so you cannot compress the speed curve yourself (so no precision is lost when expanding the curve to fit between CVs 2 & 5).