A rear-link tank in the HQ unit would relay relevent messages between A-set networks, exending a squadron net to the regiment, or a regiment net to the wider brigade/division.
A rear-link tank in the HQ unit would join it's A-set to a wider network, and relay relay relevent messages to the commander on the B set. This would extend a squadron net to the regiment, or a regimental net to the wider brigade/division.