Limitations, assumptions and approximations of Hevacomp, particularly CIBSE Gain

I am wondering if you could help point me in the right direction.

I am wanting to better understand what are the limitations of Hevacomp in terms of the underlying assumptions and approximations that the software uses. 

I am mostly interested in the CIBSE Gain tool, but if you have information about the other tools this would also be useful.

This is so that I can get a better understanding of how the software works, and the accuracy of the results.

Thanks,

Brendan