I am learning about decision tables and am confused by the colouring in the text book's mark scheme for a question I'm working on. I can not see why some of the columns have been coloured grey. Rather than adding anything to my understanding, the colouring makes me think I'm missing some important point as I can't see a commonality between the columns which have been coloured.
The Questions for which this image is the solution are: i) Complete the decision table and ii) simplify by removing redundancies.
Can anyone please explain the colouring to me?