1, %VOL: i.e., volume fraction, which indicates the proportion of CO2 molecules occupying a specified volume of gas, usually expressed in the form of a few percent. For example, 1% VOL of CO2 means that there are 1 volume unit of CO2 molecules in every 100 volume units.
2, ppm: that is, parts per million, that is, how many parts of every 1 million parts of the gas are CO2 molecules, usually used to express the proportion of CO2 concentration is very small. For example, 400 ppm of CO2 means that there are 400 parts of CO2 molecules in every 1 million parts of gas. These two units are often interchangeable, and you can choose the appropriate unit for your application.