He's probably refering to DPF (diesel particulate filter) deletes. I'm pretty sure that is what just about 95% of customers at tune shops in Europe are in there for, bypassing or deleting DPF so they don't have to deal with all the costs around it. (and the annoyances of software messing with how your car drives to "renew" the DPF randomly)

I know someone that runs a tune shop in my small town and I'd say it is about 99.99% of his customers that are in for a DPF delete or bypass.