Because everyone here has played Aigis /sarcasm.
In FKG 3star units have a bronze border, 4star has a silver, 5star has gold and 6star have rainbow.
In Idol War Z, R units have a silver border, SR has gold, SSR has rainbow.
In X-Overd, UC units ranks are in bronze front, R units are in silver, SR in gold and SSR is rainbow.
To do the final break limit...?