system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there an 'X' in permanent marker on my dog's lower belly?

Been meaning to ask this for a long time.

My dog was rescued from the Humane Society and one of the first things we saw was a small, neat green X, that looked like it was drawn in thin permanent marker.

We've asked a ton of people and we've answers anywhere from the "that means the dog was spayed" to "that means she's a 'good behavior' dog to the Humane Society" to "it means she's been given a 'don't euthanize' order."

But no one can tell me for sure. Anyone know?
gpt
It's a tattoo that indicates spayed/neutered. 

They put them on unclaimed or animals that might escape or be released. They mostly use them on cats, as cats are commonly let outside without identification.