Units of troops always have a leader, and commonly include a standard bearer who carries the unit's banner, and a musician such as a drummer or warrior with a horn.
The leader model is always placed in the middle of the front rank of the unit. If a unit does not include a specific model to represent its leader, then the leader is assumed to be the model in the middle of the front rank. The unit's standard bearer, and musician must also be placed in the front rank where possible. When the unit turns round to face the side or rear its leader, together with standard bearer and musician, are automatically rearranged into the new front rank.
As we shall see later a unit can also be joined by a hero or wizard, in which case these models are also arranged into the front rank alongside the leader, standard bearer and musician. Don't worry about this for now, we'll discuss special rules for heroes and wizards later on.