All subjects have a Band 2 cut-off and if your raw mark is below that cut-off then you get a Band 1. It is determined by the same committee that determines the cut-offs for the other Bands. In fact, in my experience, we spent most time discussing the Band 5/6 cut-off followed by the Band 1/2 cut-off. Not that we didn't give an appropriate amount of discussion and thought to the other band cut-offs but they were the ones that probably got the most attention.
These raw cut-offs can be in single figures for some subjects or can be in mid-20s for others but from talking to other people in other subjects I would think that in pretty well all subjects a raw mark in the 30s would be a Band 2 (could be wrong on that as I haven't spoken to every 'judge' every year of course).