ksb switch

[1990CTD]

Ok guys I need help. I'm having new new pump put on my 1990 cummins. The original was leaking bad. So I got a rebuilt one from a intercolled dodge. I've heard change the ksb? Whats it do? Where is it? And do I have to have it? I have a guy putting the pump on for me and was looking for advise on it. Help.