The "linking" is the process that produces a linked database, out of the unlinked multi-output Unit Process of the ecoinvent database. The linking follows a series of steps; some of them are common for all system models, while others are specific to each system model. You can read more about the linking rules in the Chapter 14 of the Data Quality Guidelines.
Data Quality Guidelines
In the consequential system model, all by-products, also not material for treatment by-products, are moved to the input side, with a negative sign. Then, all inputs and outputs will be linked to the corresponding market activities (unless an Activity Link has been specified for the exchange). In the case of the consequential system model, the level of technology of the suppliers as well as other constraints on suppliers will be taken into account at the moment of the linking.