Your Imperial Guard army represents a detachment drawn from an Imperial Guard Company. The detachment is led by a Command HQ. The HQ and all of the specialised heavy weapon squads (Mortar, Fire Support and Anti-Tank) in the detachment form a Command platoon consisting of the Command HQ unit and up to five heavy weapon squads.
The whole platoon counts as a single HQ choice on the force organisation chart. But although bought as one HQ choice, the Command HQ and heavy weapon squads may fight as separate units, deploying in different parts of the battlefield and fighting as independent units. The platoon, however, counts as a single 'choice' when deploying units or rolling for reserves.
Any Imperial Guard squad within 12" of a Command HQ may use the Leadership of the Officer in the Command HQ for all Morale or Pinning tests unless the Officer is falling back. Note that you always use the Officer's Leadership value, even if a Commissar is attached to the Command HQ, as long as the Officer is still alive (see the rules for Commissars below...).