In the context of LCA, a "system" is defined as any set of devices (units) that carry out one or more specific industrial operations with a determined function; it is delimited by appropriate physical boundaries concerning the environment and has relationships of exchange characterized by a series of inputs and outputs.
To perform the inventory analysis of a system, it is necessary to first define the individual component operations, or "unit processes": each of them receives inputs from the processes upstream, while its outputs will feed those downstream, according to the actual production scheme. In steady-state conditions, the behavior of a process unit is independent of both the processes upstream that provide inputs and those downstream that receive the outputs. Consequently, this behavior can be analyzed without reference to the other processes within the overall system. This consideration is very important because global systems are not always analyzed, but it is often appropriate to limit the analysis to parts of them by defining productive subsets. The definition of subsets and individual process units essentially depends on the level of detail achievable with the available information, the collected data, and the time constraints imposed for completing the study.
