Ordering a new rear end

[Veryfine69]

I just ordered a new 9" rear end for my mini tubbed 69 Camaro. I thought it would be fairly simple, once I got the overall measurment, just call it in, answer a few questions, pay for it, then wait for delivery. A couple questions in, the guy stumped me, he asked if I wanted the drive centered, or the housing centered. I had no idea what to say. Is this a concern to have it one way or the other?

Thanks
Morley