Hi the original fault was flagged as the entry flow to the DPF was becoming blocked this is measured by the DPF pressure sensor which has one pipe connected to the intake flow and the other is connected to the out flow, both pressure measurements are sent to the ECU so it can monitor the flow of gas and particles that are being trapped by the DPF, as soon as the readings on the in flow start to drop below threshold status it starts a regeneration to burn off the soot, problem is it only burns enough to start operating again within the prescribed parameters therefore some particles are left in situ, thats where the Catalan comes in as it dissolves the remaining particles in between regeneration so the frequency for that process becomes less as the holes in the core are less blocked as when new.
The ultimate DPF technology was designed by the Italians who were experimenting with Microwave technology for city diesels this gave a 98% burn rate and resulted in near minimum Nox but the project was binned when the European Union went anti diesel, and of course all the nobs in Brussels know whats best for us.