The center serving is the serving that goes right where the arrow sits in the grove and protects the string from the barrel, the nock and the claw mechanism that holds the string back. This wears out and will need to be replaced from time to time, depending on how much you shoot. My old shop would charge about seven bucks to unstring my CB and put a new center serving on. On my old Horton, the cables only cam into contact with the wheels and I had no serving touch them. On my Supermax the string went around the wheels once before hooking onto the cables and at that point on the string it was served. I never wore it out, but one time had to have it replaced because it started to unravel on me and it needed to be there to protect the string. If your string is OK and has no broken strands then I would just get it reserved, if needed. But if it has lots of fraying or broken strands then you should get it replaced too. On my old Supermag I think the guy charged me 20 bucks for a new string once, can't remember though. Never wait until the string breaks to replace, you could be injured by the CB or at least break it. Where your center serving meet the arrow and barrel, turn it or twist it with your fingers to see the other side that comes into contact with the barrel. Thatthe point that will wear out first, the top side will usually always look good.