From Halopedia, the Halo wiki
The rounds consist of harpoon-like shafts and rope bags. Once loaded into the barrel of a weapon—such as an SRS99C-S2 AM sniper rifle—and fired, the shaft becomes embedded in the target and a braided monoline uncoils, pulled by the grapple. Soldiers can then use this line for climbing.
During the Onyx Conflict, Kurt Ambrose ordered Linda-058, Frederic-104, Mark-G313, and Holly-G003 to use grappling rounds to scale the dome structure located in a Forerunner city to set up lookout positions and zip lines.
List of appearances