HI What is Copper Special Coin for?

You can either sell it or use it.

https://subeta.net/explore/coinmachine.php

Going there will allow you to use the coin and you can get an item, sP or a stat boost for your active pet. Or sometimes the coin pops back out, which just means you can try again for an item.

thanks

If you want to navigate there instead of following a link, the subway to the Lower West Side is in the lower left corner of the Centropolis map.

The Coin Machine accepts any coin with "special coin" in the name. There are coins in Subeta that don't have the word "special" and they will not work.

thank you

You are welcome