## 1 Introduction  
The European Green Deal (COM (2019) 640 final), adopted by the European Commission in 2019, has set a vision of how to tackle climate and environmental challenges and to achieve a resource-efficient, sustainable and climate neutral economy by 2050 [1]. Transport accounts for a quarter of the European Union's greenhouse gas emissions and these continue to grow, being the only economic sector whose GHG emissions are 32% above the 1990 levels despite the various mitigation efforts taken. The European Green Deal has set the objective to achieve a reduction of 90% of the GHG emissions from transport CO2, relative to 1990 levels. The European Commission released on 14 July 2021 the proposal for the revision of the RED II under the \"Fit for 55\" package of legislative proposals in order to implement the new ambitious 2030 climate target. The new text require the increase of the target for the use of renewable energy to 40% and the increase of the level of renewable energy in transport by setting a 13% greenhouse gas intensity reduction target for 2030. This is a change in the approach for decarbonising the transport sector, focussing on a target based on greenhouse gas emissions instead of an energy-based target. The greenhouse gas emissions savings shall be calculated for biofuels and biogas, for renewable fuels of non-biological origin and recycled carbon fuels and for renewable electricity, by multiplying the amount of these fuels or renewable electricity supplied to all transport modes by their emissions savings. In the European Union (EU), biofuels are eligible for counting towards the target set by the EU's Renewable Energy Directive (RED), as long as they meet the required levels of GHG savings compared to the fossil fuel comparator and comply with sustainability criteria required by the RED. Therefore, an EU Member State is allowed to claim the amount of biofuels resulting from biomass feedstocks processed with fossil fuels in a common process (co-processed biofuels) used in their country towards their renewable energy targets. Most biofuels currently used around the globe, i.e., FAME (Fatty Acid Methyl Ester) biodiesel and bioethanol, are produced in stand-alone facilities. These biofuels are typically blended into regular fossil fuels at low percentages, up to 7 vol% in European standard diesel fuel (sometimes referred to as B7), or at up to 5% or 10 vol% ethanol in regular gasoline (also known as E5 or E10 gasoline, respectively). Conversely to co-processed fuels, the blending percentage of FAME and ethanol can always be easily determined, on the basis of the differences in molecules when compared to fossil fuels. Drop-in biofuels, defined as liquid bio-hydrocarbons that are functionally equivalent to petroleum fuels and are fully compatible with existing petroleum infrastructure [2] receive a lot of attention, as they can be used in internal combustion engines as such and with no limits on the blending levels. Drop-in biofuels can be produced through several processes that include oleochemical processes (hydroprocessing of lipid feedstock); thermochemical processes (thermal conversion of biomass through pyrolysis or gasification to intermediates followed by catalytic upgrading/hydroprocessing); biochemical processes (biological conversion of sugars or cellulosic feedstock to alcohols and hydrocarbons); and hybrid thermochemical/biochemical processes (fermentation of synthesis gas/off gas followed by catalytic reforming) [2]. Co-processing is an opportunity for the oil sector to contribute to decarbonisation objectives. Some sectors will continue to rely on liquid hydrocarbons even in the medium term, and co-processing is an economic option on short-term for some sectors that are difficult to decarbonise today, until renewable energy sources will be replacing fossil fuels. Despite co-processed fuels started to be put on the market, a tracking methodology to detect the final bio-content in the fuels produced in the oil refineries is still under definition at EU level.