TACPAD
From Halopedia, the Halo wiki
The TACPAD[1] is a piece of United Nations Space Command equipment. It may be a weapon, but is most likely a tactical database system used for analyzing combat information. One UNSC Marine stated: “How the foxtrot does it track anything? Whatever—I guess that’s why I carry an em eh five instead of a TACPAD.”, in reference to a Covenant Plasma Pistol's tracking feature.
This comment indicates a TACPAD might be used for "tracking" a target, but not for actually engaging it. It might also be related to a "Data Pad", which is a small hand-held computer mentioned in the Halo novels.
