Understanding Society User Support

I am keen to create an after housing costs in BHPS but am unclear what is included in the variable xphsg (gross monthly housing costs). I would like to know whether this (i) includes or excludes housing benefit and (ii) includes or excludes the principal payment for those with mortgages. If the principal is included, is there a way to deduct this from xphsg (i.e. a way to split mortgage payments into interest and principal)?

Thanks in advance for any help.