It is only given to troublemakers who break the rules. Instead of being on the finger, the tracking cuff is a metal bracelet on the wrist that cannot be taken off.
After Zane and Tally take the cure pills, they are given these cuffs because they scaled a building, which doesn't conform to normal pretty characteristics. The cuffs monitor the wearer's movements and can listen in on conversations, so Tally and Zane try to get them off by starving themselves and later by expanding the metal with a hot air balloon fire.
After Tally and Zane got their cuffs, they became insanely cool in New Pretty Town, so everyone requested fake cuffs from the hole in the wall. It has all of the same abilities as an Interface ring, including the ability to ping people.