Above is a sketch I did to help keep track of the different roles a dog may have on a dog sled team. Terms such as lead, swing or wheel dog would be slung around like moose stew at a checkpoint. As a volunteer I needed to learn quickly. I noticed that a musher will move his dogs around depending on the situation. An example would be when the teams crossed from Shaktoolik to Koyuk, across 50+ miles of sea ice, often times the musher would use an older, calmer dog to lead the team across the ice and not the normal leader. The few times I witnessed this, the dog used to lead the team across the ice had been a wheel dog, but, not always.
The number of dogs you can start (Willow) and finish (Nome) with on the Iditarod is also a frequent question—16 (start) and 6 (finish) (from the 2012 Iditarod rule book). Not sure if the number you can finish with changes from year to year or not.