BioARC (Advanced Recon Commando) Troopers were specially trained Clone Troopers or Clone Officers who were often assigned to special missions and often trained Clone Cadets on Kamino.
They were originally trained by Jango Fett and wore experimental Phase 1 armor (which the full version wasn't yet developed; better known as the "Null Class"), to whom he wanted to have a series of specially trained Clone Officers to lead platoons, squads, and battalions into battles. (Even though they eventually served and had different purpose, as seen above.)
The ARC Troopers originally had distinguished markings of the standard Phase 1 Clone Officers, until Jedi Anakin Skywalker developed a much more organized strategy for having distinguished markings for squads and battalions, as well as having personalized armor.
Later on towards the middle of the Clone Wars Era, they eventually began to wear experimental Phase 2 armor, which led to the creation of a much more safe, secure, comfortable, and effecient version of armor.
ARC Troopers have an officer position lower than Veteran Clone Officers, but have a higher position than every other. (Standard, Senior, Elite.)
Elite Officers were inspired by the ARC position of Officer, to whom ARC Lieutenant Alpha developed.
A'den was the only ARC Trooper to have the rank of Sergeant, however, he wasn't an official ARC Sergeant and was an experimental Null Class ARC Trooper, who wore experimental Phase 1 armor.
Phase 1 ARC Troopers retired, were killed in combat, or died of growth-acceleration towards the middle of the Clone Wars Era.