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Purpose The ISO 14044 standard for life cycle assessment (LCA) provides the reference decision hierarchy for dealing with multi-functional processes. We observe that, in practice, the consistent implementation of this hierarchy by LCA practitioners and LCA guidance document developers may be limited. In an attempt to explain this observation, and to offer suggestions as to how consistency in LCA practice might be improved, we identify and compare the rationales for (and limitations of) different common approaches to solving multifunctionality problems in LCA. Methods The different prevalent understandings of specific approaches for dealing with multi-functional processes were identified, and their respective rationales were analyzed. This takes into account identifying the implicit underlying assumptions regarding the nature and purpose of LCA that support each approach. Results and discussion We identified what we believe to be three internally consistent but mutually exclusive schools of thought amongst LCA practitioners, which differ in subtle but important ways in terms of their understanding of the nature and purpose of LCA, and the multi-functionality solutions necessary to support them. These three divisions follow two demarcations. The first is between consequential and attributional data modeling approaches. The second is between adherence to a natural science-based approach (privileging physical allocation solutions) and a socioeconomic approach (favoring economic allocation solutions) in attributional data modeling. Conclusions We conclude that the ISO 14044 multifunctionality hierarchy should explicitly differentiate between attributional and consequential data modeling applications. We question the feasibility and practical utility of system expansion (currently privileged in the ISO hierarchy) in attributional data modeling applications. We suggest that ISO 14044 should also make explicit its rationale for privileging natural science-based approaches to solving multifunctionality problems and to more clearly differentiate between natural science and social science-based approaches. We also call for the formulation of additional guidance for solving multi-functionality problems, in particular for justifying the use of lower-tier solutions from the ISO hierarchy when these are applied in LCA studies. We suggest that this additional guidance and clarity in ISO 14044 will contribute to increased consistency in LCA practice and also increase the potential for users of information from LCA studies to make informed decisions as to their relevance within the context of specific intended applications.

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