An ear-tip is the universally recognized symbol of a cat who has been spayed or neutered and vaccinated. Ear-tipping is a standard part of most Trap-Neuter-Return (TNR) programs, because it's the best method to let everyone know at a glance that a cat is altered and vaccinated. 

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Ear-tipped cats can be friendly or unsocial. Ear-tipped cats also play an important role in their neighborhood by offering pest control and keeping other unvaccinated, unfixed cats from moving in. Ear-tipped cats are sometimes “indoor/outdoor” cats who live in the area, and they can be expected to make their way back to their main caretaker/residence soon. Unless injured or ill, ear-tipped cats should never be taken to the shelter.