Biofuels continue to play a role in strategies for decarbonization, especially for transportation. However, to determine whether a particular biofuel pathway can achieve decarbonization, its life cycle greenhouse gases (i.e., carbon intensity) must be calculated. The two most important steps in the biofuel life cycle are feedstock production and conversion. When a feedstock is derived from a waste or residue, it can lead to very low carbon intensity for the resulting fuel. Consistency in how waste or residue designations are made is critical, especially when carbon intensities are used in a regulatory context. We develop a taxonomy for feedstock classification, supported by a decision tree for the designation of feedstocks, which may be classified as primary products, co-products, by-products, or wastes. These designations are critical for determining the allocation method by which carbon intensity estimates for feedstocks are calculated under policies such as California's Low Carbon Fuel Standard. The decision tree is designed to include two possible questions as the fifth step, differentiated by the inclusion or exclusion of an economic threshold. An economic threshold is common for distinguishing between co-products and byproducts in biofuel policies. Based on the resulting decision tree, several feedstock candidates are tested, and the results analyzed through a SWOT (Strengths, Weaknesses, Opportunities, Threats) analysis to determine if there are specific benefits or drawbacks of the proposed decision tree. The results from the feedstock tests show that the choice of the fifth question does not affect carbon intensity estimates for the selected test cases. However, depending on the economic value of a feedstock, a feedstock could be deemed a byproduct instead of a co-product based on an economic threshold. Choosing to include an economic threshold in the decision process will result in more feedstocks designated as by-products, rather than primary products, thus attributing them a lower carbon intensity.

Key words: Biofuel, Feedstock classification, Life cycle assessment, Allocation method, Carbon intensity, Low carbon fuel standard