Why does my Nikon 3100 Instruction Guide say to use a 5' usb cable?

The instruction guide says if you are using a "third party usb cable" to use a five foot cord, and attach the smaller of the two supplied ferrite cores. I already have a usb cable that is about two feet long to use, and it has a little knobby thing near the small end that looks like the supplied ferrite cores. Would it be a bad idea to use this cable to transfer my pictures from the camera to the computer? And what does it mean when it says "third party usb cable"?

Third party means a company thats not nikon.(in this situation). Just use the cable that you have.

Read again. That may be the longest you can use not necessarily exactly 5 feet. You can use whatever length you have as long as the plugs fit.

If you have the ferrite core, you can go ahead and use your cable. Don't worry about what the manual says.