Rex's unit represented a larger-than-usual squad of clone troopers belonging to the 501st Legion, commanded by Clone Captain Rex throughout the Battle of Umbara. This particular squad's mission centered around capturing the Umbaran airbase; however, numerous clones perished (killed) during the assault on the base. Consequently, ARC trooper Fives alongside clone trooper Hardcase were compelled to utilize Umbaran starfighters to neutralize the opposing Umbaran forces.
The initial mention of this squad occurred during the introductory narration of the Star Wars: The Clone Wars episode entitled, "Plan of Dissent," although their on-screen debut took place in the preceding episode, "The General."