Communications between Imperial Guard units on the battlefield are dealt with by troopers trained to use special comm-links. The size and appearance of a comm-link depends on its place of origin but in game terms they all have the same effects.
If a Command HQ or Command Section has a comm-link, then one squad per turn that also has a comm-link may use the Leadership value of the Officer, no matter where they are located on the battlefield (ie, they don't have to be within 12" of the Officer as would normally be the case). You may choose to use the comm-link at any time (eg, when an eligible squad is about to take a Leadership test or if the command unit is in reserve).