From Halopedia, the Halo wiki
Unlike other Covenant military units, the Warhost is purely a temporary grouping of soldiers and other units created on a temporary basis for the purpose of accomplishing a given objective. The Warhost will remain formed until the objective is fulfilled and the unit is disbanded, though the Covenant will repeatedly use a warhost in an operation until it is destroyed. Evocati assigned to a Warhost would not answer directly to the Warhost commander, but instead elect a representative from within their ranks (known as Evocati Delegatus) who would swear a binding oath to obey their commander and act as an authority on specialist subjects.
Types of Warhost
A Siege Host is an armoured battle group consisting of a Ogab'd-pattern anti-aircraft Wraith and Type-32 Ghost commanded by a Type-26 Wraith, alongside a small amount of infantry, a large amount of other vehicles and some air support.
In Halo: Ground Command, Siege Hosts may have the following attributes
List of appearances