A cat that has been neutered during a TNR appointment has their left ear "eartipped." 1/4" of the ear is removed while under anesthesia during surgery. This lets the world know "I am fixed" and allows a TNR trapper to see there's no need to take the kitty to the clinic. Therefore, they can be released from their trap immediately. One thing to know is that many eartipped cats could be owned cats so if you come across a friendly eartipped cat, don't assume it is a community cat. Be sure to check for a microchip as well!