This is a group that is not seen at all Teq events, because most of the time it will not be needed. However, it's good to know what it is, incase it should be needed in an emergency.
Emergency Response Team, or ERT, is a group that, 99% of the time, is formed in the Zerg and stays there. The only time that they leave the Zerg are for battery phases, clearly, and if another defense is in serious need of help.
If that is the case, then that defense will call for ERT to come help them at their position, and ERT will come clear the area and then return to the Zerg after the team that called for them is okay again.
The possible need for an ERT can also be totally avoided if you have eight people at South Boat, and seven a North Boat.