Some units have an option in the Empire Army list to purchase one or more support units, called Detachments. Units that can purchase Detachments have the Regimental Unit special rule. Units that can be fielded as a Detachment have the Detachment special rule. Some units have the option to be either a Regimental Unit or a Detachment, though they cannot be both, so make sure your opponent is aware of which units in your army are Regimental Units, and which are Detachments, when you place them on the battlefield. The following rules apply when selecting Detachments:
Detachments cannot be taken without a Regimental Unit.
A Detachment’s size can range from a minimum of five models to a maximum of half the number of in the Regimental Unit (rounding up).
Detachments can purchase any equipment upgrades listed in their army list entries, but may not include a standard bearer, musician or unit champion.
The points cost of a Detachment and any upgrades it has taken, counts towards the points total of the same unit category as the Regimental Unit they are purchased with.
No Regimental Unit may have more than two Detachments.
As long as the Regimental Unit is not fleeing, their Detachments that have at least one model within 3" use the Regimental Unit’s Leadership, unless it would otherwise be higher. If a Regimental Unit has any of the special rules listed below, they confer the same special rule onto all of their Detachments whilst they have at least one model within 3":
Whether or not a Detachment is Steadfast is determined by their Regimental Unit. This means that if a Regimental Unit is either not engaged in combat itself, or is engaged in combat and is Steadfast, then all of its detachments are Steadfast, even if fighting an enemy with more ranks. If the Regimental Unit is engaged in combat and is not Steadfast, then none of its detachments can be Steadfast, even if fighting an enemy with less ranks.