I've also added it to the guide:
I've tested this and finished it. The condition that must be met is as @ElliotLH says, in that your team must control one (and only one) of a two objective sector. Other specifics--the other objective may either be controlled by the enemy, or be in a neutral state. You must be on the objective when killing. I've tested it by killing enemies who were contesting the objective, and I received credit. I also tested it on enemies who were just outside the objective, and it appeared to count. The name of the kill type that appears in the kill feed is called, "Objective Defended". If you control all objectives within the sector, most easily notable on a one-objective sector, you'll receive a "Sector Defended" kill. For this requirement, you want the "Objective Defended" kill. Needless to say, it must be on conquest mode.
