The Diesel Particulate Filter system uses a catalyst fluid to operate. The soot particles in the exhaust are collected in the filter and then burnt off into carbon dioxide at regular intervals when called for by the ECU. This is a dealer only top up system. I think the fluid is expensive, I've heard people talking of hundreds of pounds to get it refilled. Some manufacturers apparently need a complete tank as it's not refillable, you're looking at about £1200 for that by all accounts. Some systems don't use a catalyst fluid but rely on an engine cycle equivalent to a 40 mile non stop drive at over 40 mph. During this time, the engine overfuels in order to raise the exhaust temperature to about 600 C to regenerate. The catalyst fluid systems will regenerate at about 300 C so do not need the overfuelling cycle.