**1. Introduction**

Life cycle assessment (LCA) is supposed to be a standardized methodology to measure the life cycle impacts of products or services. LCA is currently ruled by ISO 14040:2006 and ISO 14044:2006 [1,2]; these standards have been the basis of the LCA methodology for the last two decades. Nevertheless, in the scientific community, some experts wonder if the detail presented in these standards is enough to guide LCA practitioners in practice [3–5].

One of the most debated problems in LCA is the so-called "multifunctionality" issue (or commonly, "allocation") [6–8]. Multifunctionality issues need to be dealt with when different product systems share a process, e.g., manufacturing processes delivering more than the studied product, or end-of-life activities providing both waste management service and a recovered or recycled product. In these cases, apportioning environmental burdens among the co-products, or rather co-functions, becomes necessary. According to ISO 14044:2006, multifunctionality should be solved by using the following three-level hierarchy [2]:


The same hierarchy applies also to "open-loop" recycling, i.e., when a material is recycled as a different product because it is no longer suitable to replace the original product directly. Only in such open-loop recycling, ISO 14044:2006 provides further guidance on the third level of the hierarchy, where physical properties (e.g., mass) are preferred to economic value, which in turn is preferred to the number of subsequent uses of the recycled material [2].

The existence of the ISO's multifunctionality hierarchy should avoid the use of inadequate approaches, e.g., determined by the interests of the stakeholders or the ones of the study's commissioner [3]. Nevertheless, the apparent lack of sufficient guidance has fed different implementation practices [9]. Consequently, although most LCAs claim compliance with the two ISO standards, practitioners have applied different allocation procedures in LCAs assessing the same or similar products [10]. Since the choice of the allocation method typically affects the outcome of the LCA significantly [6,10–13], this problem has led to different conclusions and therefore low reliability and robustness of the LCA results [14]. Moreover, due to the lack of a shared view in the LCA community, some authors decide not to follow the ISO hierarchy (see [15]), while other authors select the allocation method based on their subjective decision (see, e.g., [16] and [17]). Other researchers choose allocation methods that are "commonly" applied in similar case studies in the literature (see, e.g., [18]), others calculate also an average allocation parameter considering common parameters (e.g., [19]) or others use "conservative" allocation methods that provide the highest impacts (e.g., see [20]).

This article presents a literature review on the main practices and debates on using ISO 14044:2006 recommendations to solve multifunctionality problems. A critical literature review on multifunctionality methodology development was combined with quantitative analysis of current multifunctionality practices, and a bibliometric review based on citation network analysis (CNA). The quantitative analysis was performed by a text-mining process in 532 multifunctional case studies found in the literature.

The CNA was used to identify the main knowledge flow on multifunctionality in LCA, also known as "the main path". Tools and software based on the "main path" method are used for many applications: tracking the evolutionary trajectory of a science field or the development of a specific technology, or the evolving changes of legal opinions of courts [21,22]. The "main path" was investigated to detect the historical origins of the different practices currently present in the literature and their underlying theories. The use of such a tool overcomes some limitations of the traditional systematic reviews conducted so far on this topic, which were based on "human" selection of the articles (e.g., through criteria such as the number of citations).

In the literature, the definitions used to characterize the multifunctionality issue are not harmonized. For this reason, we provided Appendix A reporting the definitions used in this review to distinguish the different types of products, multifunctional processes, modeling approaches and system expansion approaches.
