From Halopedia, the Halo wiki
The Collar accessories are compatible with MJOLNIR Mark V. Aesthetically, all Collar configurations share a common design, in which they are extended to cover more of the wearer's pectoral region. The Collar/Grenadier chest piece extends around the neck to the far upper back, forming a "collar", hence the nomenclature.
Spartan-II Jorge-052 from NOBLE Team employed the up-armored variant of the Collar/Grenadier chest piece. After the Human-Covenant War, the Collar/Breacher configuration was employed by Hazel-A302 during her time at Outpost Discovery. Additionally, the INDOMITABLE-class Mjolnir, being a recreation of Jorge-052's customized armor, also featured the up-armored variant of the Collar/Grenadier chest piece.
All Collar armor types can be unlocked in the Halo: Reach's Armory through the game's purchase system via the use of cR. In Halo: The Master Chief Collection, the Collar armor types are attainable in the Season 1: Noble progression system, with a new variant, Collar/Grenadier [UA] [R], being introduced in Season 5: Anvil. This extension of the player models' aesthetic appearance does not affect their hit boxes, however, it can make for a slightly more visible target, especially in the case of the Collar/Grenadier [UA] variant.
Halo 5: Guardians
List of appearances