Ecosmart back is for a tank that has more than 2 pumps with some on each end and the back as well.
a representation of 3 pumps varying power over time in an ecosmart mode.
From the manual
The most important thing to remember is that when using EcoSmart modes, you assign slave pump modes based specifically on where they are positioned. If a pump is placed opposite the master pump, it goes in the orange mode. If it is placed on the same side as the master, it goes in the white mode. If the pump is placed on the back side of the tank, it goes in the turquoise mode.
With Mobius the colors have been replaced with names.
unless things have changed there are 2 ecosmart modes
Tidal Swell mode (TSM) creates a harmonic balance reminiscent of the changing flow conditions found in nature. Over the course of several hours, the flow pattern will go from very chaotic to calm, followed by a detritus-clearing surge at the very end of the cycle. The flow direction will change direction every cycle. In this mode your master driver will be purple and your slave driver will flash white/orange/teal and indicate its speed by the LED strip.
Nutrient Transport mode (NTM) creates circulation in a two-phase program. The first phase uses wave motion to stir up detritus. The second phase follows with a surging motion to move the detritus into the overflow and out of the aquarium. This mode promotes maximum health, nutrient export and increased growth for sensitive corals such as SPS