Here's why:

* CO is the chemical formula for carbon monoxide, a molecule composed of one carbon atom and one oxygen atom.

* 12 refers to the specific isotope of carbon used. Carbon has several isotopes, with the most common being carbon-12. The number before the element symbol indicates the mass number (protons + neutrons) of the isotope.

So, 12CO specifically indicates carbon monoxide containing the carbon-12 isotope.