It's almost self-explanatory. The DCI is a rank above DI. He will have a DI working 'for' him on cases and the DI is answerable to him. My guess is that he 'steers a desk' most of the time, because there are not many cases that require an officer of that rank to be involved in the investigation. And when you get to higher ranks of detective than that, I suspect that their rank only serves to have them giving press conferences, their actual involvement not being great :) Mind, I did call a chief constable as a witness once; he had asked to be taken out on a patrol, as he was new to the area, when suddenly the car was called to an affray and he found himself in the middle of it. That was not involvement that he wanted. To his credit, he did arrest somebody, who must have been amazed at the sight of such a fancy uniform!