Reduced center timing chain?

[GreenF350]

I'm planning on changing the timing set on my wife's Jeep Cherokee with the 4.0L. I am well educated and and capable to do the job (or at least I thought I was) but I have a question and I cannot find the answer anywhere. Autozone lists an "Optional Performance Item" that is a Timing set with .005" reduced center and .010" reduced center. I have no idea what this means. Makes me feel kind of stupid for not knowing since it has to be something simple and I am a mechanic by trade (not automotive anymore), but I guess I have never run accross that before. Can anyone enlighten me? Thanks!
-Aaron