From Halopedia, the Halo wiki
Keepaway Mode is a gametype in the Strategic Options Pack DLC for Halo Wars alongside the Tug of War and Reinforcements modes, and was later included in Halo Wars: Definitive Edition. Keepaway serves as the Halo Wars adaptation of "Capture the Flag". Teams battle it out to capture a free-roaming Forerunner Sentinel to score points before the other team. To win, players must capture three to five sentinels to claim victory.
The player must send a unit to the sentinel and command the unit to pick up the sentinel. The unit will then have to be protected for a set amount of time. If the unit survives, the player gains a point. If the unit is killed, the flag resets and the point is not earned. Unlike the FPS games, units that have the flag can fire at enemies.