Further theory on performance of product systems
The reference flows must take into account all those properties (whether obligatory, positioning, or market-irrelevant), which have been previously identified as part of a difference in performance. It is not just the obligatory product properties that determine the amount of substituted product or the interaction with other product systems. For example, the ease of cleaning a beverage container (a non-market relevant property) can influence the amount of cleaning agent.
The reference flows must include any difference that involves additional processes in one or more of the analysed product systems. It may be relevant at this stage to include the complementary (or linked) products that are used together with the product, but which may not be part of the original product definition. An obvious complementary product is packaging, but also additional products needed for maintenance, replacements, waste treatment, or recycling of raw materials must be included in the description, whenever these processes are planned or can be foreseen to be necessary. The notion of avoiding differences between compared systems is, in fact, parallel to the procedure for handling co-products by adding or subtracting processes until the systems deliver only the desired, comparable output.
It is important to consider whether customer behaviour may be affected differently by the different product alternatives. This is especially relevant when studying consumer products and may often significantly affect the outcome of the study. Changes in consumer behaviour should be included in the reference flow.