Vox-caster
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 vox-caster 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 vox-caster, then one squad per turn that also has a vox-caster 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 vox-caster at any time (eg, when an eligible squad is about to take a Leadership test, even if the Command unit is in reserve). The benefits of standards and trademark items are not conveyed through the vox-caster!
Errata — Chapter Approved: Codex: Imperial Guard Q&A (v1.0): With a Master-Vox, every eligible unit with a vox-caster uses the Officer's (potentially boosted) Leadership for its own tests — but special rules such as Iron Discipline or a Trademark Item still do not convey down the vox link, only the raw Leadership value does.