around here the shelter vets and low cost clinics are SUPPOSED to tat id on altered cats, its a number, when entered into a shelter or veterinary database comes up with the animals info. this is primarily for the feral control program in the area, there were a number of accounts of recapture on ferals who had already been spayed and it was costing money to put them all under and open them up only to find they were already done...
they do the tat inside the ear though so its realy easy to spot.
they also use it to track the movment of feral populations.
my fave so far was a vet who always wanted to be a tat artist growing up, he dabbled for a long time but whent into veterinary medicine as a career instead of tattoo...
whenever he did a spay or neuter (with the owners permission of course) hed do little pretty and unique ones) like a heart, or a star...they take just as long as a regular tattoo to heal so why not.
of course i wouldnt have my animal put under FOR a tattoo...but if there already under for a spay/neuter and were going to be tatted for id anyway, why not make it something cute?! lol