Understanding Society User Support

I am currently analysing Wave 3 of the Understanding Society dataset and am looking to construct an income distribution at the household level. The variable on which the income distribution shall be constructed needs to be before tax and benefits are accounted for and so I am using the variable,“c_fihhmngrs_dv” (gross income: month before interview). Specifically I would like to know the exact definition of this variable, specifically whether this variable includes benefits or not? Otherwise any suggestion of the correct variable to use for the analysis would be much appreciated.

It would also be useful to know why for some observations in this variable, there are negative values such as -8000?

Many thanks in advance.