Hi,
Thanks for your inquiry.
It is true that BME680 and BME688 are the same based on MOX technology. The only difference is the gas output resistance range in the ubnit of Ohm. BME680 has typical resistance range that can be used for measuring indoor air quality (IAQ), total VOC and equivalent CO2 concentration in the unit of ppm.
BME688 has larger output resistance range that can be trained to detect specific gas. Different gases such as coffee, cheese, alcohol, etc. has different output resistance pattern under certain heating profile inside BME688. By using AI Studio SW available on website users can generate model for specific gas. This is so called "a gas scanner function". Please see the video clip at https://www.youtube.com/watch?v=LbDWbdlbD8Q for more information.
Thanks.