From Halopedia, the Halo wiki
Collar is a group of chest pieces for use in conjunction with MJOLNIR Powered Assault Armor. The Collar accessories are compatible with MJOLNIR Mark V. Aesthetically, all MJOLNIR/Collar variants have one thing in common; they are extended so they cover more of the wearer's pectoral region and extend around the neck to the far upper back, forming a 'collar', hence the nomenclature. 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 up armored [G] Variant.
List of appearances