I still don't get the point of the two cords. Why does one have the third grounding prong and one doesn't? The sales person said the one cord was longer, but it only seems slightly longer. The only real explanation in the documentation is on how to remove the plug from the MagSafe adapter and put it on the other cord, but there is no real explanation as to when you should use one or the other plug. I just thought it was strange so I can understand why the OP might be confused.
I don't know, if the word "extend" has that many meanings in English, but I understood it as extension cable when I bought my iBook. I seldom used it, but with my MBPs, I use it more often. It extends the reach by 1.5m, which can be sometimes helpful.To extend the reach of your power adapter, replace the AC plug with the AC power cord.
I still don't get the point of the two cords. Why does one have the third grounding prong and one doesn't?