The 940M has 2 versions. (I know, it is a mess)
GM107 and GM108.
GM107 is uses PCI-E 16x, while the GM108 uses PCI-E 8x (version 3.0 on both), and they are other variations in the specs.
They both support GDDR5 and DDR3 (not GDDR3).
Have a look at this:
GeForce 900 series - Wikipedia, the free encyclopedia (go to 940M on the table).
You will notice that the 940M GM107 model is a slightly reduced 950M
I think the GPU in the Surface Book is a GM108 version of the 940M, because it is on PCIe 3.0 x8, downclocked a bit, with elements of the GM107 model and on GDDR5.