This Element introduces the Disambiguating Project (DP) about the units of selection. By DP, the authors mean the thesis that the expression 'units of selection' refers to at least three non-co-extensional functional concepts: interactor, replicator/reproducer/reconstitutor, and manifestor of adaptation/type-1 agent. They present each concept and demonstrate the necessity of their isolation, because each of them responds to a distinct question about the units of selection, and these distinct questions are not always posed in combination in today's biological research. They further apply the framework to the analysis of the debates concerning the Evolutionary Transitions in Individuality (ETI) and argue that the DP interprets the ETI better than any project rejecting the three meanings of 'units of selection.' Thus, they claim that the differentiation between at least these three functional concepts is fundamental to clarify some conceptual confusions in biology, which rest on the conflation of these distinct meanings.