I was reading the heterotic string section of this thesis desertation by Luboš Motl, since I think I now understand the Type IIA Matrix String Theory. The only thing I knew about Type HE Matrix Strings, however, was:
That It has a gaugegroup of $O(N)$.
From the paper, I can see that the Type HE Matrix Strings, in order to be consistent, need some sort of "extra terms" (wrt the BFSS Lagrangian Density/SuperYM Lagrangian Density ) in order to exist in 11Dimensional spacetime.
Now,

What is the intuition behind this; intuitively, why does the absence of these "extra terms" cause such problems in 11Dimensional spacetime. Given the amount of intuition behind Type IIA Matrix Strings, I expect that there may be some intuition in Type HE Matrix Strings too ?

Where does the new gauge group of $O(N)$ come from when the gauge group of BFSS itself, was $U(N)$ all along.