Dear Jona,
Thank you for raising this question, we will revise the FAQ to make it clearer.
The cut-off and APOS system models do not interfere with each other, the linking of each happens independently. What they have conceptually in common is the attributional approach, i.e. they depict the average situation at a given point in time.The linking algorithm of cut-off and APOS reflects this average situation by including all suppliers of a product into the supply mix (market activity), regardless of whether they produce the product as a byproduct and can thus not respond elastically to demand changes and regardless of the technology level.
The allocation between reference product(s) and by-products is both in cut-of and APOS based on product prices, i.e. economic allocation.
You can find explanations of each system model here, for cut-off and for APOS. Feel free to come back in case of further questions, either here or drop us an email to support@ecoinvent.org
Kind regards,
Florentine Brunner
Data Analyst, ecoinvent