Highly specialized, but worth an explanation:"LT up, grab C3 please."
This means a teammate has grabbed a Long Tom, and will be helped out if you choose a unit carrying C3. This extends its range beyond what its own radar can reach, and in many cases can show units that would've been otherwise hidden. Also in special situations, It can show targets in the 1,800+m range of the LT which are hittable in the right terrain.
Typically, just having the C3 is good enough, but if people are willing to coordinate, C3 can chain out across an entire map.