Understanding Society User Support

I am currently writing my Masters-Thesis about the UK labour market and whether sexual orientation has an impact on various labour market variables.

I would like to answer this question by using the data from the UKHLS survey and have a specific question regarding income: there is a variable called c_inc1lab and I am wondering how it is measured, what it includes exactly and what time period it covers.
By analyzing this variable I would like to find out whether there are differences in labour income depending on sexual orientation. If there are other variables that I could use instead, I would be grateful for that information as well.