There is no requirement as to how a manufacturer meets the emissions requirement. The requirement is the emissions. Some opted to use DEF from the start and do not have a DPF. Some opted to go the DPF route and now are adding some DEF to meet Tier 4 final. I believe the standard is in grams per kilowatt-hour so smaller engines are allowed to emit more pollutants per kW-hr but since they produce fewer kilowatts, total emissions is lower. Instead of using a sliding scale, they chose to group power classes. Particulates are worst during transitions like engine accelerations. Companies going the DPF route generally are shooting for a very responsive engine and filter out the soot. NOX increases with combustion temperature. Using EGR will reduce combustion temperature at the expense of fuel economy. DEF allows high combustion temperatures but the DEF works with a catalyst to break the NOX down into nitrogen and oxygen - plain old air. Deere says their DPF/DEF combination gives a more responsive engine. CNH says using their DEF only solution is a simpler, less expensive solution eliminating the expensive DPF.
Europe has a much higher density of Diesel engines than North America and is already concerned about emissions, especially particulates, with the Tier 4 standard. Europe uses a different designation, Stage instead of Tier, and the numbering is slightly different. They are now getting ready to set Stage 5 standards for about 2019 - 2020. Engine producers want North America to follow. Japan will likely be as tough or tougher. Still a lot to shake out.
